Apr. 1903. North American Plesiosaurs — Williston. 



73 



was briefly described by me several years ago under the nam. <>t 

 Plesiosaurus gouldii. The species is referred to PUsiosaurus because 

 no better place is known for it ; in all probability it really belongs to 

 some other genus. The species was based upon several dorsal ver- 

 tebrae in fairly good preservation, one of which, the best, is herewith 

 figured. (PI. XXVII.) 



The specimen shows little compression, and its form is doubt- 

 less normal. The anterior face is rather deeply concave, cordatt in 

 outline, with a small neural depression above. The anterior zyga- 

 pophyses are spout-like, the notch between them not extending 

 further than the middle of the articular surfaces. The spine is 

 rather short and small. The transverse processes are compressed, 

 springing in part below the neural canal from the base of the arch. 

 The body is compressed in the middle, forming an obtusely rounded 

 surface below. About midway on the sides, below the lower root of 

 the transverse processes, the side is pinched in, with a small vascular 

 foramen at the bottom of the depression. 



The vertebra described probably belongs near the sacrum, its 

 measurements are as follows: 



Width of anterior end of centrum 



Vertical diameter, same end 



Length of centrum 



Height of vertebra 



Expanse <>f transverse processes 



Width of neural canal 



Expanse of anterior zygapophyses 



no mm. 



75 



79 

 '75 

 175 



30 



50 



PROPODIAL BONES OF YOUNG PLESIOSAURS. 



It is an interesting fact that isolated propodial bones of young 

 plesiosaurs are not at all rare in the Kansas chalk ; no more so indeed 

 than are bones of the adult animals. I have seen more than a score 

 of such, and four or five are now preserved in the museum of the 

 University of Kansas ; there are many others in the Yale museum. 

 Four of these bones are shown in PI. XXIII; a fifth one, more immature 

 than any of those, is figured in PI. XXII, Figs. 1-4. All such 

 bones are composed of more dense tissue than is observed in adult 

 bones. Especially is the structure dense in the youngest specimi n 

 here figured. In this specimen the head of the bone is not at all dif- 



