AND AVES OF NORTH AMERICA. .% 



The caudal vertebras 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 artic- 

 ular 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 coossified 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 decurved 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 coossification of cervical vertebrae 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 for- 

 amina 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 prob- 

 able 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 ascer- 

 tained, 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 



