mammalia (excepting only the tridactyl sloths which have nine), the series is exactly seven ; and so strict is the adherence to this rule, that even the short and stiff neck of the whale, and the long and flexible neck of the camelopard, are formed out of the same elementary number; the vertebræ in the former instance being extremely thin and anchylosed together, and in the latter greatly elongated. Reptiles possess only from three to eight cervical vertebra ; birds, approaching in this more nearly to the present species, but still falling greatly short of it, have from nine to twenty-three*, the number being the greatest in the swan. The ichthyosaurus appears to have possessed about eighteen cervical vertebræ; in fishes the ribs commence almost immediately behind the head.
The views of Geoffrey de St. Hilaire, that nature in the organization of the animal frame has caused the sternal portion to shift its position along the vertebral column, seem to derive an important corroboration from the structure of this animal; but it is remarkable that whereas the sternum holds a mean position in quadrupeds, and is thrown forward in fishes and backwards in birds, yet its position in this instance assimilates the plesiosaurus less to fishes, though destined to move in the same element†, than to birds, and exhibits at the same time a very wide departure from the type of the Saurian tribe. Although the number of the cervical vertebrae is thus unexampled, yet the length of the neck is nearly rivalled by another of the reptile class, namely the land tortoise. The length is in this case concealed by the anterior extension of the shell; the neck, however, notwithstanding its elongation, has only eight vertebra; The general proportions of the tortoise, its length of neck, shortness of tail, and the smallness of its head, are in some degree analogous to what we observe in the plesiosaurus; but the structure of the head and teeth of the latter, and its want of shell, entirely negative the idea of its being