H I C H G95 head, which is provided with four pairs of barbels. Mouth without lips. Nasal duet without cartilaginous rings, penetrating the palate. One median tooth on the palate, and two comb-like series of teeth on the tongue. Branchial apertures at a great distance from the head ; the inner branchial ducts lead into the oesophagus. A series of mucous sacs along each side of the abdomen. Intestine without spiral valve. Eggs large, with a horny case provided with threads for adhesion. Genera : Mijxine and Bddlostoma (Hag-Fish). Subclass IV. Leptocardii. Skeleton membrane-cartilaginous and notochordal, rib- less. No brain. Pulsating sinuses in place of a heart. Blood colourless. Respiratory cavity confluent with the abdominal cavity ; branchial clefts in great number, the water being expelled by an opening in front of the vent. Jaws none. This subclass is represented by a single family (Cirro- stomi) and by one or two genera (Branchiostoma and Epigionichthys) ; it is the lowest in the scale of fishes, and lacks so many characteristics, not only of this class, but of the vertebrata generally, that Haeckel, with good reason, separates it into a distinct class, that of Acrania. The various parts of its organization have been duly noticed in the former parts of this article. (A. c. G.) ICHTHYOSAURUS (from i x Ovs, a fish, and o-aCpos, a lizard), a genus of extinct reptiles, the species of which are the only known representatives of the order Ichthyopterygia. Upwards of thirty of these have been described, all of Mesozoic age, the genus so far as is certainly known appearing for the first time in the Liassic formation where it most abounds, continuing throughout the Oolitic, and disappearing before the close of the Cretaceous period. In Britain its remains have been found in greatest abundance in the Lias of Lyme Regis, although it occurs more or less commonly throughout the whole of that formation from the south of Dorsetshire through Somerset and Leicester to the Yorkshire coast. They are found in rocks of similar age in France and Germany ; and Sir Edward Belcher ob tained remains of a Liassic ichthyosaur from an island in 77 16 N. hit., one of many proofs that in Mesozoic times a comparatively warm climate must have prevailed within the Arctic Circle. Remains of true ichthyosaurs have not yet been found on the American continent, although Professor Marsh lately (1877) described portions of the skeleton of a saurian obtained from strata of Jurassic age in the Rocky Mountains which seems to have differed from Old World ichthyosaurs chiefly in the absence of teeth, the jaws being " entirely edentulous and destitute even of a dentary groove." For the reception of this form Professor Marsh proposes to institute a new order Sauranodonta ; but it has been suggested, on the other hind, that Sauranodon should rather be regarded as the type of a new family of the old order Ickthyopterygia. Owing to the comparative abundance and excellent preservation of ichthy >saurian remains, the hard parts of those creatures have been studied under exceptionally favourable circumstances, and much has thus been learnt of their structure and, by inference therefrom, of their life history. They were large marine reptiles, measuring in some instances 30 feet in length, and somewhat re sembling in appearance the dolphins of the present day. Like these they were air-breathers, and must therefore have come to the surface to breathe, although being cold blooded they were no doubt able, like the aquatic saurians of our own period, to remain much longer under water than the warm-blooded Cetacea. The ichthyosaurian head was large, and was prolonged into a more or less elongated snout, certain species rivalling in this re -pect the gavial of the Ganges. The brain cavity, on the other hand, was remarkably small. The eyes were enormously large, the orbit in Ichthyosaurus plat yudon the largest known species having been found to measure 14 inches in long diameter. This huge eyeball was protected by a ring of bony sclerotic plates similar to those found in rapacious birds and in turtles and lizards of the present day. The jaws, which in Ichthyosaurus platyodon have been known to measure 6 feet in length, were rendered still more formidable by their array of strong, conical, pointed teeth, numbering in some instances over one hundred and eighty, and placed not in distinct sockets, as in the crocodile, but in a common alveolar groove. These, as they became worn, were replaced by a succession of young teeth, which budded up at the base of the old. The neck in the ichthyosaurus was extremely short, and not marked by any constriction. The vertebrae resembled those of fishes in being deeply biconcave. The tail was long and tapering; in many specimens this organ has been found to be fractured at about a fourth of its length from the extremity, and, as the vertebrae of the same region seem to have been flattened vertically, Professor Owen regards it as probable that these reptiles were provided with a tegumentary caudal fin like that of the Cetacea, only vertical instead of horizontal ; the sole evidence of the presence of such a horizontal fin in extinct whales would be the horizontally flattened condition of the last caudal vertebrae, should any of these chance to be preserved. Ichthyosaurians were provided with two pairs of limbs in the form of paddles, which externally must have borne considerable resemblance to the anterior limbs of dolphins and other Cetacea. They differed, how ever, very markedly from these in the possession of a bony apparatus, stretching, in the case of the front pair, from one shoulder joint to the other, on which the anterior paddles were supported. This "scapular arch," according to Professor Owen, resembled, " in the number, shape, and disposition of its bones," the same parts in the Australian Ondthorhynchus, a mammal which leads, as Ichthyosaurus did, an aquatic life, obtaining its food at the bottom of lakes and rivers, but having to rise frequently to the sur face to breathe. The hind limbs were in almost all the species much smaller than the pair in front. The skeleton of each of the paddles consisted mainly of a large number in some species exceeding a hundred of small poly gonal bones arranged in more than five closely packed longitudinal rows, the whole covered with skin, and form ing a highly elastic organ of locomotion. The ichthyo.saur was provided with slender ribs along the vertebral column from the anterior part of the neck to the tail ; a sternum, however, was wanting, the abdominal walls being strength ened by the development of transverse arcuated bones. As no trace of horny scales or bony scutes has ever been detected in connexion with those reptilian remains, it may be assumed that these sea-saurians, as they have been called, were, like the Cetacea of the present day, covered with a smooth or wrinkled skin unprovided with any of those dermal appendages. From a study of their bony structure it may be inferred that these huge aquatic reptiles inhabited the opt,-n sea, occasionally visiting the shores, where their powerful paddles enabled them to crawl on land, and where, like seals, they probably loved to bask in the sunshine. That they were predatory in their habits the tyrants indeed of Mesozoic seas might be inftr ed from our knowledge of their jaws and teeth; and this is amply confirmed by an
examination of the lull-digested contents of their stomachs.
