The single-headed ribs are short, proving that the body was slender and doubtless cylindrical, more like that of a snake. The tail was not only enormously elongated, but it was also compressed into a flat and effective propelling organ in the water. This flattening of the tail is apparent from the skeleton, with its elongated chevrons below and spines above, and it is also proved by the fortunate preservation of the extremity of the tail of one specimen, showing not only the impressions of the scales in the matrix, but also the outlines that the soft parts had in life. To quote from Lortet, in translation: “The tail was covered wholly with small scales, regularly hexagonal in shape, shining and nacreous, larger on the under side than above. The upper border of the tail was surmounted by a broad crest, extending to its extremity, and composed of large, oval scales.” The body doubtless was wholly covered with scales, though it is not probable that the caudal crest continued along the back. [Illustration: FIG. 64.—Life restoration of _Pleurosaurus_.] The limbs begin to show an aquatic adaptation, though not very pronounced. They are much shorter and smaller than are those of land-crawling reptiles; and the bones of the second series, that is, the radius and ulna, tibia and fibula, are relatively short, the beginning of adaptation to water habits. It is very probable that the feet were webbed, though the fifth digit, as usual, is shorter than the fourth. Doubtless on land the creature moved about in a serpentine way, for it could not have progressed very rapidly by the aid of its legs alone. The hind legs are longer than the front legs, and they were connected firmly with the body by means of a sacrum. The number of vertebrae in the neck is only five. The number of dorsal vertebrae is forty-three, a larger number than is known in any other air-breathing vertebrate with legs. Upon the whole, these lizard-like, almost snake-like pleurosaurians present some very curious adaptations to water life. In water they were doubtless speedy, swimming in serpentine undulations, with the small legs for the most part folded against the body and only of occasional use. Doubtless, too, had the pleurosaurs lived longer in geological history, they would have become quite snake-or eel-like, just as have some modern salamanders. In all probability the pleurosaurs lived habitually in fresh-water, perhaps visiting the shores for refuge, or for the hatching of their young. That they were not on the way toward a terrestrial snake-like body is evident from the flattened tail, and especially the crest of scales above; the tail was like that of the sea-snakes of the present time. _Pleurosaurus_, then, affords the solitary instance among reptiles of aquatic adaptation by the diminution of both front and hind extremities and the acquisition of a snake-like body and snake-like habits. CHAPTER XI SQUAMATA