ICIITHYOSAURUS. The name (from the Greek ichthys, a fish, and sauros, a lizard) was devised to indicate the closer affinity of the Ichthyosaur, as compared with the Plesiosaur, to the class of fishes. The Ichthyosaur (fig. 68) is remark able for the shortness of the neck and the equality of the width of the back of the head with the front of the chest, impressing the observer of the fossil skeleton with a conviction that the ancient animal must have resembled the whale tribe and the fishes, in the absence of any intervening constriction or neck.
This close approximation in the Ichthyosaurs to the form of the most strictly aquatic verte brate animals of the existing creation, is accompanied by an important modification of the surfaces forming the joints of the back-bone, each of which surfaces are hollow, leading to the inference that they were originally connected together by an elastic bag or "capsule" filled with fluid—a structure which prevails in the class of fishes, in the Labyrinthodonts and a few extinct aquatic rep tiles, in the existing perenni branchiate Batrachia, but not in any of the whale or porpoise tribe.
With the above modifica tions of the head, trunk, and limbs, in relation to swimming, there co-exist corresponding modifications of the tail. The bones of this part are much more numerous than in the Plesiosaurs, and the entire tail is consequently longer ; but it does not show any of those modifications that characterize the bony support of the tail fin in fishes. The numerous caudal vertebrae of the Ichthyosaurus, gradually decrease in size to the end of the tail, where they assume a compressed form, or are flattened from side to side, and thus the tail, instead of being short and broad as in fishes, is lengthened out as in crocodiles.
The very frequent occurrence of a fracture of the tail, about one-fourth of the way from its extremity, in well preserved and entire fossil skeletons, is owing to that proportion of the end of the tail having supported a cutaneous and perishable caudal fin.* The only evidence which the fossil skeleton of a whale
would yield of the powerful horizontal tail-fin characteristic of the living animal, is the depressed or horizontally-flattened form of the bones supporting such fin. It is inferred, therefore, from the corresponding bones of the Ichih,yosaurus being flattened in the vertical direction, or from side to side, that it possessed a tegumentary tail fin expanded in the vertical direction. The shape of a fin composed of such perishable material is of course conjectural, as is the outline in fig. 68. Thus, in the construction of the principal swimming organ of the Ichfityosaltrus we may trace, as in other parts of its structure, a combination of mammalian (beast-like), saurian (lizard like), and piscine (fish-like) peculiarities. In the great length and gradual diminution of the tail we perceive its saurian character ; in the tegumentary nature of the fin, unsustained by bony fin-rays, its affinity to the same part in the mammalian whales and porpoises is shown ; whilst its vertical position makes it closely resemble the tail fin of the fish.
The horizontality of the tail fin of the whale tribe is essen tially connected with their necessities as warm-blooded animals breathing atmospheric air ; without this means of displacing a mass of water in the vertical direction, the head of the whale could not be brought with the required rapidity to the surface to respire ; but the Ichthyosaurs, not being warm-blooded or quick breathers, would not need to bring their head to the surface so frequently or so rapidly as the whale ; and more over, a compensation for the want of horizontality of their tail fin was provided by the addition of a pair of hind paddles, which are not present in the whale tribe. The vertical fin was a more efficient organ in the rapid cleaving of the liquid element, when the Ichthyosaurs were in pursuit of their prey, or escaping from an enemy.