254 PALEONTOLOGY OF ILLINOIS. 



that described as P. Qoughi, (McCoy, Brit. Palaj. Foss., p. 042, 643.) 

 Thus associated, we have two generic groups which appear to be 

 respectively defined by the following characters : 



Caecharopsis. — Principal cusps very strong, erect, compressed in 

 front, rounded behind or sublenticular in transverse section ; broadly 

 expanded at base so as to occupy nearly the entire lateral diameter of 

 the base; lateral angles sharp and deeply and regularly creuulated; 

 extremities occupied by one or two more or less slender, conical late- 

 ral denticles, which are as isolated as is the case in typical Cladodus ; 

 coronal faces smooth or faintly striated vertically. Base in outline like 

 Cladodus, moderately produced and broadly rounded posteriorly ; extrem- 

 ities more or less sharply rounded, anterior face sinuose, terminating 

 below in a well-defined continuous basal ridge, which occupies the 

 greater extent of the anterior border, inferior surface excavated, poste- 

 ro-superior surface moderately convex, roughened possibly into pad-like 

 prominences. 



Pristicladodus. — Principal cusp very strong, erect, sigmoidally 

 recurved, lenticular in section, more compressed in front than behind, 

 rapidly converging to the apex, lateral edges sharp and more or less' 

 distinctly undulated or simple, lateral cusps relatively very strong, 

 sometimes even more massive than the median cusps, divergent, similar 

 in shape to the median prominence with which they are connected by a 

 prominent sharp curved intervening ridge ; anterior coronal surface 

 marked with sharp more or less irregular costre, which converge in the 

 intermediate crest, producing a faint denticulation, sometimes forming 

 quite strong spinose processes. The cost* in the lateral cusps ascend- 

 ing from the basal line diagonally to the inner margin of the cusps, 

 where they often form a spinose edge or become obsolete; the posterior 

 face is generally less strongly costate, perhaps, in some forms or indivi- 

 duals, smooth. The base generally like Cladodus, posterior expansion 

 occupied above by a more or less distinct convexity, broadly rounded 

 behind, extremities rounded or truncated, anterior margin sinuose, late- 

 rally more or less broadly rounded into the extremities with distinct 

 protuberances occupying the anterior angle either side of the mesial 

 depression and connected by a low ridge ; inferior surface moderately 

 excavated, and in some forms at least the posterior border is deeply 

 notched near the median line. 



Perhaps the base is generally narrower antero-posteriorly in the for- 

 mer genus than it is in Pristicladodus, the anterior marginal border 

 forms an uninterrupted ridge, while in the latter group it is sinuate at 

 the median line, with well-defined protuberances in the anterior basal 

 angles. The coronal region is still more diverse in the two groups, the 

 present one being distinguished by the strong cuspidate lateral cusps, 



