270 University Geological Survey of Kansas. 



In some specimens this shelf is somewhat groove-like, while in 

 others it is broad and flat. Near the anterior end superiorly 

 there are two condyles, of which the posterior is large, irregular 

 in outline, and slightly convex. Professor Hay is inclined to 

 think that two forms can be determined from these condyles, as 

 he says : 27 "I believe that the two species maybe identified from 

 the condyles of the maxillary. At least, these condyles are quite 

 different in the two species which I have been able to examine, 

 X. thaumas and X. molossus. Fig. 2 represents the maxillary of 

 the former species, fig. 3 that of X. molossus. From these fig- 

 ures, it will be seen that in X. thaumas the posterior condyle is 

 notched behind, while that of P. molossus [should be X. molossus] 

 is excavated in front. It appears, too, that the condyle is more 

 extended longitudinally in X. thaumas, more transversely in X. 

 molossus.^ I am inclined to think that Professor Hay laid too 

 much stress on this and the anterior condyle in the determina- 

 tion of his species, as a glance at the series of drawings of these 

 parts on plate XLI will show that there was quite as much va- 

 riation in these condyles as there is in the teeth, as shown 

 above. I cannot be so sure about the variation of the anterior 

 condyle, as it is broken away in many of the specimens. How- 

 ever, the constancy or inconstancy of this one minor character 

 is of little specific value. 



The anterior condyle, mentioned above, is oval in outline, 

 convex, and occupies the extreme anterior part of the laminar 

 portion of the maxilla, being thus well separated from the pos- 

 terior condyle. The two condyles are more in line with each 

 other antero-posteriorly than in Ichthyodectes . Just in front 

 of the posterior of these condyles, the thickness of the maxilla 

 becomes suddenly less and forms an abutment, against which 

 the posterior portion of the premaxilla rests. In front of this 

 the bone is laminar and supports more than one -half of the 

 premaxillary bone. The posterior extremity is very thin, and 

 in some specimens it is nearly straight, while in others it is 

 curved strongly upward. 



27. Zool. Bull., vol. II, No. 1, p. 34. 



