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.]

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.

[Illustration: Fig. 3. A Pterodactylus.]

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 i