Fig. 1.
The Ichthyosauri (Figure 1) must have been very large, seven or eight feet being the ordinary length, while specimens measuring from twenty to thirty feet are not uncommon. The large head is pointed, like that of the Porpoise; the jaws contain a number of conical teeth, of reptilian form and character; the eyeball was very large, as may be seen by the socket, and it was supported by pieces of bone, such as we find now only in the eyes of birds of prey and in the bony fishes. The ribs begin at the neck and continue to the tail, and there is no distinction between head and neck, as in most Reptiles, but a continuous outline, as in Fishes. They had four limbs, not divided into fingers, but forming mere paddles. Yet fingers seem to be hinted at in these paddles, though not developed, for the bones are in parallel rows, as if to mark what might be such a division. The back-bones are short, but very high, and the surfaces of articulation are hollow, conical cavities, as in Fishes, instead of ball-and-socket joints, as in Reptiles. The ribs are more complicated than in Vertebrates generally: they consist of several pieces, and the breast-bone is formed of a number of bones, making together quite an intricate bony net-work. There is only one living animal, the Crocodile, characterized by this peculiar structure of the breast-bone. The Ichthyosaurus is, indeed, one of the most remarkable of the synthetic types: by the shape of its head one would associate it with the Porpoises, while by its paddles and its long tail it reminds one of the whole group of Cetaceans to which the Porpoises belong; by its crocodilian teeth, its ribs, and its breast-bone, it seems allied to Reptiles; and by its uniform neck, not distinguished from the body, and the structure of the backbone, it recalls the Fishes.
Fig. 2.
Another most curious member of this group is the Plesiosaurus, odd Saurian (Figure 2). By its disproportionately long and flexible neck, and its small, flat head, it unquestionably foreshadows the Serpents, while by the structure of the backbone, the limbs, and the tail, it is closely allied with the Ichthyosaurus. Its flappers are, however, more slender, less clumsy, and were, no doubt, adapted to more rapid motion than the fins of the Ichthyosaurus, while its tail is shorter in proportion to the whole length of the animal. It seems probable, from its general structure, that the Ichthyosaurus moved like a Fish, chiefly by the flapping of the tail, aided by the fins, while in the Plesiosaurus the tail must have been much less efficient as a locomotive organ, and the long, snake-like, flexible neck no doubt rendered the whole body more agile and rapid in its movements. In comparing the two, it may be said, that, as a whole, the Ichthyosaurus, though belonging by its structure to the class of Reptiles, has a closer external resemblance to the Fishes, while the Plesiosaurus is more decidedly reptilian in character. If there exists any animal in our waters, not yet known to naturalists, answering to the descriptions of the "Sea-Serpent," it must be closely allied to the Plesiosaurus. The occurrence in the fresh waters of North America of a Fish, the Lepidosteus, which is closely allied to the fossil Fishes found with the Plesiosaurus in the Jurassic beds, renders such a supposition probable.
Fig. 3.
Of all these strange old forms, so singularly uniting features of Fishes and Reptiles, none has given rise to more discussion than the Pterodactylus, (Figure 3,) another of the Saurian tribe, associated, however, with Birds by some naturalists, on account of its large wing-like appendages. From the extraordinary length of its anterior limbs, they have generally been described as wings, and the animal is usually represented as a flying Reptile. But if we consider its whole structure, this does not seem probable, and I believe it to have been an essentially aquatic animal, moving after the fashion of the Sea-Turtle. Its so-called wings resemble in structure the front paddles of the Sea-Turtles far more than the wings of a Bird; differing from them, indeed, only by the extraordinary length of the inner toe, while the outer ones are comparatively much shorter. But, notwithstanding this difference, the hand of the Pterodactylus is constructed like that of an aquatic swimming marine Reptile; and I believe, that, if we represent it with its long neck stretched upon the water, its large head furnished with powerful, well-armed jaws, ready to dive after the innumerable smaller animals living in the same ocean, we shall have a more natural picture of its habits than if we consider it as a flying animal, which it is generally supposed to have been. It has not the powerful breast-bone, with the large projecting keel along the middle line, such as exists in all the flying animals. Its breast-bone, on the contrary, is thin and flat, like that of the present Sea-Turtle; and if it moved through the water by the help of its long flappers, as the Sea-Turtle does now, it could well dispense with that powerful construction of the breast-bone so essential to all animals which fly through the air. Again, the powerful teeth, long and conical, placed at considerable intervals in the jaw, constitute a feature common to all predaceous aquatic animals, and would seem to have been utterly useless in a flying animal at that time, since there were no aërial beings of any size to prey upon. The Dragon-Flies found in the same deposits with the Pterodactylus were certainly not a game requiring so powerful a battery of attack.
The Fishes of the Jurassic sea were exceedingly numerous, but were all of the Ganoid and Selachian tribes. It would weary the reader, were I to introduce here any detailed description of them, but they were as numerous and varied as those living in our present waters. There was the Hybodus, with the marked furrows on the spines and the strong hooks along their margin,—the huge Chimera, with its long whip, its curved bone over the back, and its parrot-like bill,—the Lepidotus, with its large square scales, its large head, its numerous rows of teeth, one within another, forming a powerful grinding apparatus,—the Microdon, with its round, flat body, its jaw paved with small grinding teeth,—the swift Aspidorhynchus, with its long, slender body and massive tail, enabling it to strike the water powerfully and dart forward with great rapidity. There were also a host of small Fishes, comparing with those above mentioned as our Perch, Herring, Smelts, etc., compare with our larger Fishes; but, whatever their size or form, all the Fishes of those days had the same hard scales fitting to each other by hooks, instead of the thin membranous scales overlapping each other at the edge, like the common Fishes of more modern times. The smaller Fishes, no doubt, afforded food to the larger ones, and to the aquatic Reptiles. Indeed, in parts of the intestines of the Ichthyosauri, and in their petrified excrements, have been found the scales and teeth of these smaller Fishes perfectly preserved. It is amazing that we can learn so much of the habits of life of these past creatures, and know even what was the food of animals existing countless ages before man was created.
There are traces of Mammalia in the Jurassic deposits, but they were of those inferior kinds known now as Marsupials, and no complete specimens have yet been found.