VERTEBKATES. 257 



relatively of less lateral extent, the antero-iuferior protuberances more 

 strongly developed and generally, though not always, more closely 

 approximated; a deep narrow notch interrupting the posterior' border, 

 and the postero-superior surface broadly convex, with indications of a 

 broad, low mesial pad. The latter teeth range from .11 to .30 inch in 

 lateral diameter. 



Without authentic specimens exhibiting all the intermediate phases, 

 it is not possible positively to assert the specific identity of the forms 

 above described. But, considering the intimate characteristics which 

 they in common possess, it seems highly probable that their individual 

 peculiarities may be attributable to the variable expression of the denti- 

 tion of different parts of the jaws. 



The elegant unique fossil first noticed above is distinguished by super- 

 ficial characters which widely remove it from any form with which we 

 have had opportunity to make comparison. In the continuity of the 

 coronal prominence, and in the form of the basal portion, it bears much 

 resemblance to the form described by Prof. McCoy under the name 

 Pristicladodu8 Goughi, from the lower Carboniferous schists of Ireland. 

 It is distinguished from that species, however, not only by its small 

 size, but more especially by the even lateral angles of the cusps, the 

 denticulatious being restricted to the coronal crest intervening between 

 the principal and lateral cusps, (although in the instance of the variety 

 P. armatus these denticulations encroach far up the lateral border of 

 the lateral cones, retaining their distinct spinose character,) and which 

 bear a closer relation to the peculiar indentations observed in the above 

 named species than they do to the crenulated edges of Carcharopsis 

 dentatus (McCoy sp.) from the Carboniferous limestone of Derbyshire. 

 Indeed, as previously expressed, there seems to be sufficient reasons for 

 doubting the generic identity of the two species described by McCoy, 

 and however disinclined we might be to refer the present form to an 

 intimate relationship with C. dentatus, the same objections would not 

 obtain in the case of P. GougM, with which latter form there exist more 

 prominent features in common, and which seem to constitute sufficiently 

 distinctive peculiarities to justify the recognition of their distinct gene- 

 ric character. 



Position and locality. — Xot common in the lower fish-bed of the Kiu- 

 derhook, also in the upper fish-bed of the same formation ; Burlington, 

 Iowa. 



