GEOLOGICAL SURVEY OF THE TEEEITORIES. 335 



differs from those of the two preceding species in having a much more 

 slender shaft. The Pneumatarthnis peloreus, Cope, established on ver- 

 tebrae, may be an ally. 



Peotostega gigas, Cope. — This fossil includes many parts of the 

 endo- and exoskeleton. The bones of the former have a radiating ossifi- 

 cation, which terminates in many cases in digitations of their margins. 

 These margins, especially of the vertebral and marginal bones, are 

 exceedingly attenuated, not being thicker than paper. The vertebral 

 has an obtuse median keel. The marginals have no inferior lamina 

 and receive the extremity of the rib. The ribs have a wide, radiate, 

 lined expansion, extending from the position of the tubercle round and 

 beyond the head. The phalanges are long and flat, and the extent of 

 the fore-limbs could have been little or nothing short of fifteen feet. 

 Found near Butte Creek, Southwest Kansas. 



CYNOCBECUS, Cope. 



Proc. Amer. Philos. Soc, 1872, January. 



Established on metatarsal and caudal vertebrse of a tortoise of uncer- 

 tain, but in any case peculiar afiinities. The caudal vertebrae are not 

 anterior ones, almost lacking diapophyses, but are long and slender, 

 and the articular faces singularly incised. The form had a tail more 

 elongate than the snapping tortoise, and different from it in details of 

 composition. 



Cynocercus incisus. Cope, {loc. cit.) — A species about the size of the 

 Mississippi snapper, Macrochclys lacertina, from near Butte Creek. 



SAUEOPTERYGIA. 

 POLYCOTYLUS, Cope. 

 Trans. Amer. Philos. Soc, 1869, 34 ; Hayden's Kept, Survey, Wyoming, 1871, 386. 



As a detailed account of this genus has been already given in the 

 report on the Geology of Wyoming, loc. cit., I will not repeat it here. 

 From this the characters which separate this genus from Pleslosaurus 

 may be derived, as follows : 



First. The deeply biconcave and very stout vertebral centra. 



Secondly. The tibia broader than long, resembling those of Icthyosau- 

 rus. 



Thirdly. The coalescence and depression of the cervicals. 



Fourthly. The continuity of the neural arches. 



Fifthly. The continuity of the diapophyses of the caudals. 



PoLYCOTYLUS LATiPiNNis, Cope, {loc^cit, p. 36, PI. I, Figs. 1-13.) — 

 The powerful extremital i)ieces indicate a body to be propelled of not 

 less than usual ^proportions. If this be the case, the number of dorsal 

 vertebrae is considerably greater than in the species of this order in 

 general, and approaching more the Icthyosauri. I do not intend to 

 suggest any afdnity between the latter and the present genus, as none 

 exists. What the extent Of cervical vertebrae may have been is uncer- 

 tain. The caudals have probably been numerous, though not probably 

 so extended as in Elasmosaurus. The size of the species can be approx- 

 imately estimated from the proportions furnished by Owen (Eep tiles 

 of the Liassic Formations) for Plesiosaurus rostratus. The skeleton o^ 

 this species measures 11 feet 8 inches, and the dorsal vertebrae are of 

 less vertical and equal transverse diameter compared with those of the 



