THE GEOLOGICAL MIDDLE AGE. 
161 
small, flat head, it unquestionably foreshadows 
the Serpents, while by the structure of the back- 
bone, 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 propor- 
tion 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 move- 
ments. In comparing the two, it may be said, 
that, as a whole, the Ichthyosaurus, though be- 
longing by its structure to the class of Reptiles, 
has a closer external resemblance to the Fishes, 
while the Plesiosaurus is more decidedly reptilian 
m 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 occur- 
rence in the fresh waters of North America of a 
Fish, the Lepidosteus, which is closely allied to 
the fossil Fishes found witli the Plesiosaurus in 
the Jurassic beds, renders such a supposition 
probable 
