ICHNEUMON-ICHNOLOGY. seize them behind the head, where its long sharp teeth inflict a fatal wound. It scratches up the sand for the eggs of crocodiles, which it eats with great avidity. It was a sacred animal among the ancient Egyptians; the killing of it was forbidden; and individuals, for the maintenance of which funds were set apart, were objects of worship. The I. is easily domesticated, and forms a cat-like attachment to the place of its residence. It is useful in keeping houses free of rats and other vermin. It is therefore not unfrequently domesticated in Egypt, as the mangouste also is in India. This species is rather smaller, of a lighter colour, and has a pointed tail. ICHNEUMON, a Linnæan genus of insects, now constituting a family or tribe, Ichneumonida, of the order Hymenoptera, section Terebrantia. The Ichneumonida are extremely numerous. Gravenhorst's Ichneumonologia Europaa describes nearly 1650 European species, and they are equally abundant in other parts of the world. Many of them are minute, others are large insects; a few of the tropical species are amongst the largest of insects. They have the abdomen united to the thorax by a pedicle, which is often very slender. The abdomen itself is slender, and the whole form attenuated. The antennæ are generally thread-like, composed of a great number of joints, and are kept in very constant vibration. The ovipositor in some is short; in some it is very long, much longer than the body of the insect, and enclosed in a kind of sheath formed of two parts, concave on their inner surface, from which it is disengaged when about to be used, the whole then often seeming as three threads proceeding from the extremity of the abdomen. All the Ichneumonide deposit their eggs either in or on -generally in-the bodies, eggs, or larvæ of insects, or in spiders. Some of them deposit their eggs in aphides. They are thus extremely useful to the farmer and gardener. Particular species of Ichneumonida are the natural enemies of particular kinds of other insects. Thus, Microgaster glomeratus and Pimpla instigator lay their eggs in the caterpillars of the cabbage butterfly. Some species deposit only one egg in the egg or larva which is destined to afford food to their own larva; others deposit a number of eggs together. Those which have a long ovipositor use it to reach eggs or larvæ under the bark of trees, in holes of wood, &c. There are many species of I. in the Eastern U. States, which are very useful as destroyers of noxious insects. Among these may be named Pimpla atrata and P. lunator, which destroy the grubs of wood-eating insects, Ceraphon destructor and a Platygaster, which lay their eggs in the maggot of the Hessian fly, and Tachina vivida and others of the family Tachinida, which destroy the Army worm. ICHNOLOGY (Gr. science of footprints) is the name given to that section of Paleontology which treats of the impressions made on mud or sand, now indurated into rock, by the animals of the period to which the rocks belong, or by meteoric or other transitory physical forces. The actual remains of the hard portions of the animals themselves are the materials on which chiefly rests our knowledge of the former inhabitants of the globe; but of many animals we know nothing more than the more or less distinct impressions made by them as they moved over the surface of a muddy shore. And in some beds, not only is the evidence of the shore-wave preserved in the ripple-mark, and the influence of the sun's heat exhibited in the superficial cracks, but frequently the passing hail-storm, or the sudden and heavy thunder-shower, has left its impress upon them, and this so perfectly, that it is not difficult to determine, from the form of the cup-like depression, whether or not the rain was accompanied by a breeze, for, by observing the amount of difference in the sides of the cup, and the position of the highest side, the direction of the gale and its velocity may be approximately determined. Though the force or body forming the impression has been removed immediately after it has made the pressure, yet in these prints the evidences of animal life and of the activity of physical forces, have come down to us from the remotest periods. The impressions occur almost invariably on rocks that have been deposited as mud; only in a few cases have they been noticed in sandstone. Sometimes the argillaceous deposit is a thin layer between two sandstone beds; it is then difficult to obtain a clear surface in the shale; but the details are carefully preserved in relief in the natural cast on In this manner the footprints are preserved at the under surface of the superimposed sandstone. Stourton in Cheshire. These influences footprints seem to be either of the following. The The necessary conditions for the preservation of silt-bed may have formed an extensive flat shore, uncovered by the tide at each ebbing. Whatever impressions were made on this plastic surface would be baked and hardened by the influence of the sun, if it remained for a sufficient time uncovered by the water; and when the tide again flowed, the hardened mud, resisting its influence, would receive another film of sediment, which would specially deposit itself in the depressions, and thus secure the would operate more powerfully on portions of the permanence of the impressions. shore which were under water only at spring-tides. served in this manner at the present day, on the The impressions of numerous wading birds are preplastic mud which covers the flat shore of the Bay of Fundy, where the tide rises, it is said, as much as 70 feet. Both Gould and Lyell have given detailed other method is one independent of the sun's influ accounts of the process as it goes on there. The ence, where, on an ordinary muddy shore during the blown sand, and the tide, on its return, flows over recession of the tide, the depressions are filled up by a level surface, on which it deposits a fresh layer of silt. remotest known period of animal life on the globe. The study of ichnology carries us back to the The deposit from which has been obtained the fragment of the oldest known trilobite (Palæopyge), contains the borings of certain worms (fig. 1) and Fig. 1.-Annelid Borings (Arenicolites): From Cambrian Rocks. impressions of rain-drops. In strata of the same period, but a little later, series of regularly recurring groups of markings are considered by Mr Salter as having been produced by the sharp claws of crustacea in walking; while other sets he refers, with considerable show of probability, to the strokes of the bifurcate tail of an unknown crustacean as it swam through shallow water. From the American representatives of the same rocks (Potsdam sandstones), Professor Owen has described a number of impressions made apparently by different animals, to which he has given the generic name of Protichnites. The slabs shew that the animals ICHNOLOGY-ICHTHYODORULITE. made at each step 14, 16, or more impressions. They of animals passing together across a tide-receded were most probably crustacea, furnished with three estuary, to some frequented ground periodically or four pairs of bifurcating limbs, like the modern sought for food or pleasure. No animal remains king-crab. Similar impressions have been observed whatever have been found associated with them; in the Lower Silurian rocks of Eskdale in Scotland, they seem, however, to belong to forms of tortoise. and have been named P. Scoticus. The tracks of numerous annelids occur also in these rocks. They Fig. 2.-Nereites Cambrensis: exhibit the impressions of the creatures as they The slab figured is a portion of the track probably of a long-tailed Chelonian, with a stride a little over six inches. The pad of the foot was soft and smooth; the light impressions of the fore-foot were nearly obliterated by the hind-foot, which was furnished with four claws (fig. 3).. Sir William Jardine, on whose property the Corncockle quarries are, has made these tracks the subject of a valuable and elaborate monograph. In the Triassic rocks, the well-known foot-tracks of the Labyrinthodon (q. v.) occur. The earliest evidence of the existence of birds was formerly supposed to be in the argillaceous sandstones of Connecticut, which are now known to be of the Triassic age. The structure of the tridactyle feet which produced these impressions exhibits the regular progression in the number of the toe-joints from the inner to the outer toe, once thought peculiar to birds, and they have been taken as evidencing the occurrence thus early of the class, although a considerable interval elapses before the first true fossil The footprints of a small reptile had been observed a bird occurs; of but they on the sandstone of a quarry near Elgin, which have recently been shown most probably belongs to the Old Red Sandstone by Professors Cope and Measures. In 1851, it was discovered that they Huxley to be those of a were produced by a little reptile (Telerpeton Elgin- form of reptiles known as ense), whose remains were there found. And lately, Dinosauria. Immense triProfessor Huxley has referred a different set of dactyle footprints have impressions to his lately described remarkable fish- been known for many like reptile, Stagonolepis. years in rocks of Wealden age in the south-east of England. At first, they were supposed to be birds; Fig. 4.-Footprints of Dinobut a more careful examination has shown them to belong to reptiles; and the discovery in the same strata of the perfect foot of a young iguanodon, measuring 21 inches in length, and furnished with three toes, which would form a print precisely similar to the tracks so long known, shows them to have been certainly produced by the Iguanodon (q. v.). The Coal Measures of our own country and of Germany have disclosed the footprints of different reptiles. The New Red Sandstone strata abound in footprints. It was the Permian or lower division of this series that supplied, in 1828, the impressions which gave the first indication of animal life from such evidences to the mind of Dr Duncan-a man who deserves to be remembered less for his works in natural history, important though they were, than for his eminent services to his country as the founder of savings-banks. The tracks he described occur on the layers of unctuous clay which separate the beds of sandstone in the quarries at Corncockle, Dumfriesshire; they frequently are clear and delicate, as at the moment when they were impressed, and Fig. 3.-Footprints of a Tortoise: From the Permian Sandstone of Annandale. are repeated bed after bed on the fresh_tablets as they were prepared for their reception. From their number and direction, they seem to be the tracks sauria: On the Triassic Sandstones of ICHTHYODO'RULITE (Gr. fish-spear-stone), the name given to fossil fish spines, that are not uncommon in the stratified rocks. Plagiostomous fishes have their dorsal fin furnished in front with a strong bony spine. The fin is connected with the spine, and is elevated and depressed by its movement. It seems also to be employed by the fish as a defence against its larger foes. Some bony fishes have similar spines, as the Sticklebacks, Silurids, &c. The spines are most frequently unassociated with any fish remains, having belonged to plagiostomous fish, in which the spine is simply implanted in the flesh, and consequently would be speedily separated from the body of the fish when it began to decompose. The earliest certain evidence of vertebrate animals is the spines of plagiostomous cartilaginous fishes which occur in the bone bed of the Ludlow rocks, the uppermost of the Silurian deposits. Spines belonging apparently to three species have been found: they are small, compressed, slightly curved ICHTHYOLOGY-ICHTHYOSIS. and finely grooved lengthwise, and belong to the genus Onchus. Along with them have been found petrified portions of tubercular and prickly skin, like the shagreen of the shark. The Old Red Sandstone has supplied such a variety of spines as to have afforded the materials for establishing fourteen genera, and in the Coal Measures they are more numerous, belonging to no less than twenty-one genera. ICHTHYO'LOGY (Gr. ichthys, a fish; logos, a discourse), that branch of natural history which treats of fishes. Aristotle is the most ancient author having any claim to be noticed in a history of ichthyology, nor was this science much indebted to any other of the ancients. In modern times, it began to be cultivated, about the middle of the 16th c., by Belon, Rondelet, and Salviani. Towards the close of the 17th c., it made great progress through the labours of Willoughby and Ray; in the 18th c., through those of Artedi, Klein, Linné, Gronow, Brunich, Scopoli, and Bloch; in the beginning of the 19th c., through those of Cuvier and De la Cépède; whilst, more recently, Valenciennes, Müller, Agassiz, and Owen are eminent amongst many who have prosecuted the study of ichthyology with ardour and success. The name of Yarrell deserves to be particularly mentioned for his work on British Fishes. The earlier ichthyologists generally included the Cetacea among fishes. Linné removed the Cetacea to their proper place. He also placed the Cartilaginous Fishes with Reptiles in his class Amphibia, from which they have since been, by the common consent of naturalists, brought back to their place in the class of Fishes. Linné's system of ichthyology is almost as artificial as his system of botany. It is founded on the relative positions of the pectoral and ventral fins, without reference to any important point of comparative anatomy or animal economy. Other ichthyologists, both before and since, have laboured to discover a natural arrangement, to which the progress of comparative anatomy has greatly contributed, although success is still confessedly very imperfect. Even the system of Agassiz, founded on the external covering of fishes, is not wholly artificial, and is of very convenient application to fossil ichthyology. ICHTHYOSAURUS (Gr. fish-reptile), a remarkable genus of reptiles which inhabited the sea during the deposition of the Secondary strata. Like the modern Cetacea, their structure was modified to suit their aquatic life. The body was shaped like that of a fish, the limbs were developed into paddles, and the tail, long and lizard-like, was furnished, it is believed, with a fleshy fin, as in the dolphin, except that its position was vertical. The head was large, and produced into a long and pointed snout, resembling that of the crocodile, except that the orbit was much larger, and had the nostril placed close to it, as in the whale, and not near the end of the snout. The jaws were furnished with a large series of powerful conical teeth, lodged close together in a continuous groove, in which the divisions for sockets, which exist in the crocodile, were indi cated by the vertical ridges on the maxillary bone. Ichthyosaurus. The teeth were hollow at the root, sheathing the young teeth, which gradually absorbed the base of the older ones, and, as they grew, pressed them forward, until they finally displaced them. The long and slender jaws were strengthened to resist any sudden shock by being formed of many thin bony plates, which produced light and elastic as well as strong jaws. The most remarkable feature in the head was the eye, which was not only very large-in some specimens measuring 13 inches in diameter-but was specially fitted to accommodate itself for vision in air or water, as well as for speedily altering the focal distance while pursuing its prey. The structure, which thus fitted the eye so remarkably to the wants of the animal, consists of a circle of 13 or more overlapping sclerotic bony plates surrounding the pupil, as in birds. This circle acted as a sort of self-adjusting telescope, and accompanied by the extraordinary amount of light admitted by the large pupil, enabled the ichthyosaurus to discover its prey at great or little distances in the obscurity of the night, and in the depths of the sea. The neck was so short that the body was probably not in the least constricted behind the head. The backbone was fish-like; each joint had both its surfaces hollow, making the whole column very flexible. The small size of the paddles compared with the body, and the stiffness of the short neck, seem to suggest that the tail inust have been an important organ of motion. Professor Owen is satisfied that it was furnished with a vertical tail, because the vertebræ are compressed vertically, and also because the tail is frequently found disarticulated a short distance from its extremity, as if the weight of the upright tail had caused it to fall when the animal had begun to decompose. The fish-like body, the four paddles, and especially the powerful tail, would make the ichthyosauri active in their movements; and conse quently, with their predaceous habits, very dangerous enemies to the other animals that inhabited with them the Secondary seas. That their principal food consisted of fishes, is evident from the masses of broken bones and scales of contemporary fishes that have been found under their ribs in the place where the stomach of the animal was situated. The remains of ichthyosauri are peculiar to the Secondary strata, occurring in the various members of the series from the Lower Lias to the Chalk, but having their greatest development in the Lias and Oolite. More than 30 species have been discovered; they differ from each other chiefly in the form of the head, some having a long and slender snout, like the gavial of the Ganges, while others had short and broad heads, more like the common crocodile. The great repository for ichthyosaurian remains hitherto has been the Lias at Lyme Regis. ICHTHYO'SIS, or FISH-SKIN DISEASE, is characterised by a hardened, thickened, rough, and almost horny state of the cuticle, which breaks |