of the Triassic epoch.  It is greatly to be regretted that all that is known of the Triassic Reptiles antecedent to the Ichthyosauri still remains in the form of original papers, and is not yet embodied in text-books.  They are quite as interesting, as curious, and as diversified as those of the Jurassic epoch, which are, however, much more extensively known, on account of the large collections of these animals belonging to the British Museum.  It will be more easy to understand the structural relations of the latter, and their true position in the Animal Kingdom, when those which preceded them are better understood.  One of the most remarkable and numerous of these Triassic Reptiles seems to have been an animal resembling, in the form of the head, and in the two articulating surfaces at the juncture of the head with the backbone, the Frogs and Salamanders, though its teeth are like those of a Crocodile.  As yet nothing has been found of these animals except the head,—­neither the backbone nor the limbs; so that little is known of their general structure.

[Illustration:  Fig. 1.  An Ichthyosaurus.]

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.

[Illustration:  Fig. 2.  A Plesiosaurus.]