66 PALEONTOLOGY OF ILLINOIS. 



While it is not deemed improbable that the acquisition of more 

 complete evidence may lead to the discovery of still more intimate 

 relationship between PsepJwdus and typical Cochliodonts, the rela- 

 tions of the former with Psammodus are so remote as to preclude 

 the inference, which has, however, found expression, that Psephodus 

 constitutes a sort of transition connecting the Psammodonts with 

 the Cochliodonts. 



PSEPHODUS OBLIQUUS, St. J. and W. 



PL I, Figs. 1, 2, 3, 4, 5. 



Under the above term are included series of teeth represented in 

 the collections by a number of individuals, which range themselves 

 under two, perhaps three or more well-defined groups, distinguished 

 by their outline and general conformation, and which, taken together, 

 are riot considered separable into so many distinct species; on the 

 contrary, there exists strong presumptive evidence of their having 

 constituted parts of the dentition of one and the same species. 



Only the two most conspicuous forms are noticed in this place. 



One of these forms is represented by teeth which possess the char- 

 acteristics of what may be regarded the typical representative teeth 

 of the genus Psephodus, comparable with the larger and more tumid 

 teeth of P. magnus, Agass., distinguishable, however, by their more 

 symmetrical and smaller size. In general outline convolutely and 

 irregularly pentangular, moderately inrolled, outer extremity slightly 

 rounded and obliquely truncated from front outward and downward, 

 the basal portion showing a broad channel beneath the projecting 

 coronal border; anterior border slightly sigmoidally curved, forming 

 nearly a right angle with the outer margin, coronal enamel forming 

 a narrow crenulated fold above the nearly vertical-channeled base ; 

 posterior border similarly curved, but of greater extent, agreeing in 

 other respects with the opposite side ; inner margin broadly rounded 

 or obtusely angular, coronal portion well-defined from the deep, 

 somewhat produced and shallow-channeled base (in most specimens 

 the basal portion is worn away, the crown projecting beyond its 

 original support). Coronal surface moderately arched in both direc- 

 tions, traversed a little posterior of the middle by the median ridge, 

 which forms a rather prominent angulation partaking of the curva- 

 ture of the defining borders, anterior slope broader than the some- 

 what more concave and abrupt posterior declivity, which latter is 

 produced into an obtuse alation at the extreme posterior angle, be- 

 yond which extends the base terminating in an obtuse spur. Sur- 



