388 GEOLOGICAL SURVEY OF THE TERRITORIES. 



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 has not attained such 

 huge dimensions as some of the latter. These fragments do not throw 

 much light on the structure of the pelvic arch. 



The structure of the bones is, like that in the order generally, of the 

 coarsest description. There are no medullary cavities, but the medul- 

 lary cells are large, and extended everywhere in the direction of the 

 axis of each bone. 



The characters which separate this genus from Plesiosaurus may be 

 derived from the preceding as follows : 



First. The deeply biconcave, and very short vertebral centra. 



Second. The tibia broader than long, resembling those of Ichthyosau- 

 rus. • 



Third. The coalescence and depression of the cervicals. 



Fourth. The continuity of the neural arches. 



Fifth. The continuity of the diapophyses of the caudals. 



The only genus with which this genus compares nearly is the Thau- 

 matosaurus of Meyer. This is known by but a few fragments, and of 

 these but few are present in the Kansas animal. The character on 

 which I rely at present to distinguish them is the much less concavity 

 of the dorsal vertebrae in Thaumatosaurus. This is, however, not en- 

 tirely satisfactory. Thaumatosaurus oolithicus, Meyer, is from the lower 

 oolite of South Germany. 



The bones are thoroughly mineralized, and the adherent matrix is a 

 light-yellow, chalky limestone, similar to that which yielded the fine 

 fragments of the Liodon proriger. This, Dr. Leconte informs me, is 

 probably Meek and Hay den's Upper Cretaceous ISTo. 3, 'and is a higher 

 horizon than that near Fort Wallace, from which Dr. Turner procured 

 the Elasmosaurus platyurus. The specimens were all taken out under 

 the direction of W. E. Webb, of Topeka, from the same spot. From 

 every point of view there is reason to believe that they belong to the 

 same animal. 



POLYCOTYLUS LATIPINNIS, (COPE.) 



Loc. Cit., p. 36, Plate I, Figs. 1-13. 



The anterior dorsal vertebrae have the centra slightly compressed or 

 vertically oval, while the posterior are more rounded. The anterior 

 caudals appear to have been round or nearly so ; they are somewhat 

 distorted by pressure. The sides of the centrum are slightly concave 

 in the longitudinal direction ; below there is no carina, but at least two 

 venous foramina. There is another large foramen on the side of the 

 centrum, usually not far from the neural arch ; there are usually other 

 smaller foramina below this. The bases of the diapophyses are longi- 

 tudinally grooved behind, and separate a concavity of the arch in front 

 of them from one behind. In the most median the most elevated dia- 

 pophysis stands about equally on the neurapophysis and the neural 

 spine above it. The diapophyses are vertically compressed, and the cos- 

 tal articulation of the only one preserved is in the same plane. The 

 margins of the external surfaces are not coarsely striate as in many 

 Sauropterygia. The venous foramina of the distal cossified cervicals are 



