The caudal vertebræ are anteriorly quite as large as the dorsals. Two anterior caudals present on the latero-inferior part of the posterior margin, a pair of widely separated articular surfaces for chevron bones. A portion of one of the latter remains; it is narrow and sub-cylindric at the base. The diapophyses are situated on the upper part of the centrum, and are continuous with it, and without trace of suture. There are two distal cervicals, which are much smaller than the preceding. They are solidly coösified and have been broken from one anterior to them, with which they have been also anchylosed. Processes in the position of diapophyses have disappeared, while a strong infero-lateral process projects from the middle of each, similar in position to the parapophyses (or whatever they may be) of the Elasmosaurus. These processes are decanted and much thickened and rugose; they may be described as more or less elongate conic. The neural canal of these vertebrae is well marked, though small. The coössification of cervical vertebræ is a remarkable character, and very unusual. It does not seem probable that these specimens represent a diseased condition, since they are symmetrical, and the inferior surface and foramina are unaffected. The rugosity is much that of a ligamentous articulation. Their size indicates a remarkably slender neck as in Plesiosaurus, but even more so, and perhaps as elongate as in Elasmosaurus.
That the portions of an extremity alluded to, belong to the posterior, is rendered probable by the presence of part of an ilium, and by the fact that the portions of the vertebral column secured, are chiefly median and posterior. The fragments consist of the extremity of the femur, the tibia, several tarsal bones, and numerous phalanges. The whole limb is of great size compared with that of the vertebral column, and indicates powerful natatory capacity in its possessor. What the relative length of the femur may be, cannot be ascertained, as the proximal portion is wanting, but if it were like the tibia, it was characterized by stoutness rather than by length. The portion remaining is flattened, and presents distally two distinct articular faces for ulna and radius, instead of the uniformly convex outline characteristic of most of the species of Plesiosaurus. The tibia is broader than long, and not emarginate externally. The fibula is wanting. One of the tarsal bones is a flat unequally hexagonal disc, of less thickness than the tibia and the tarsals which appear to connect with it. One of the latter is transverse parallelogrammic, with three faces of broad plane articulations and the outer rounded in section. Another tarsal or metatarsal is a parallelopipedon, except that one extremity presents two faces meeting at a right angle. Another is similar, but oblique, i. e., rhombic in section; one of the longitudinal angles is also prolonged.
Of the phalanges there are individuals from three series. Portions of flat bones, perhaps, belonging to the pelvic arch, indicate, as do all the other pieces, that the bony structure in Polycotylus is more massive than in Elasmosaurus, if the only known species
