VERTEBRATES. 4Q7 



those forms, from which we have provisionally recognized its specific 

 distinctness. 



Teeth irregular or eccentric in outline. Crown moderately thick, 

 irregularly arched along the crest, sharp, acuminate, apex submedian ; 

 the concave crown-face is but slightly depressed, lateral angles strongly 

 produced, subacute, inferior border strongly and eccentrically arched 

 downward and rounded below, deflected to the lateral extremities 

 above, and margined by four or more imbricating folds, which are well- 

 defined below, attenuated in the lateral borders on nearing the extrem- 

 ities, where they are usually obsolete or broken away ; convex face 

 semielliptical in outline, in elevation much less (one-third to two-thirds) 

 than that of the opposite face, rather strongly arched laterally, sharply 

 rounded at the lateral angles, inferior border moderately prominent, 

 gently beveled to the base, gently arched downward from one or other 

 extremity and suddenly curved upward near the opposite extremity, 

 and bordered by two or three narrow imbrications continuous with the 

 basal belt of the concave face. The coronal surfaces in the specimens 

 before us are delicately ornamented with vertniculose strire, and along 

 the crest diverging stria? appear in the triturating surface. The base is 

 too indifferently preserved to determine its entire form; it appears to 

 have been thin, tapering to the broadly rounded extremity, which proba- 

 bly extended but little beyond the extreme basal margin in the concave 

 face, forming a broad convexity in the opposite face. Greatest lateral 

 diameter of crown .22 inch, elevation of convex face .12, elevation of 

 concave face about .18 inch. 



The basal band of the concave face is not shown in its lateral exten- 

 sion, where it was probably very attenuated before reaching the lateral 

 angles. The eccentric outline of the concave face is due to the irregu- 

 larly arched basal borders, as also that of the opposite face. In this 

 particular these teeth bear a remarkable resemblance to some of the 

 distorted teeth found associated with and referable to P. striatus, N. and 

 TV., in the Upper Burlington limestone. But in the case of the present 

 form, we have been unable to trace such intimate specific features with 

 either of the two forms described from the St. Louis limestone, while 

 the identity of the specimens from widely separated localities, would 

 seem to furnish additional evidence of their specific distinctness. In 

 the direction of the basal folds, and the rather prominent convexity of 

 the median region of the convex crown-face, there is a less or more 

 remote resemblance to P. spatulatus ; but there exist no such intimate 

 features in common with the latter form, such as enable the probable 

 identification of the abnormal individuals of the Upper Burlington form 

 from those above referred to, and which the teeth under consideration 

 more closely resemble than they do either of the forms with which they 

 are associated. 



