72 



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 cancellous cells 

 are large, and extend everywhere in the direction of the axis of each bone 



The characters which separate this genus from Plesiosaurus may be 

 derived frpm the preceding, as' follows : 



First. The deeply biconcave and very short vertebral centra. 



Second. The tibia broader than long, resembling that of Ichthyosaurus. 



Third. The coalescence and depression of the cervicals. 



Fourth. The continuity of the neural arches. 



Fifth. The continuity of the diapophyses of the caudals with the centra. 



The only genus with which this one may be here compared 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 vertebra 

 in Thaumatosaurus. This is, however, not entirely satisfactory. T.ooUtliicus, 

 Meyer, is from the Lower Oolite of South Germany. 



The bones of the specimens of Pohjcotylus arc thoroughly mineralized, 

 and the adherent matrix is a light-yellow chalky limestone, similar to that 

 which yielded the fine fragments of the Liodon proriger. 



POLYCOTYLUS LATI1HNMS, Cope. 



The anterior dorsal vertebrae have the centra slightly compressed or ver- 

 tically oval, while the posterior arc more rounded. The anterior caudals 

 appear to have been round, or nearly so; they are somewhal 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 longitudinally grooved behind, and separate a concavity 

 of the arch in front of them from one behind. In the most nearly median, the 

 most elevated diapophyses stand about equally on tbe ncurapophysis 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 



