VERTEBRATES. 415 



FlSSODUS TRICUSPID AT us, St. J. and W. 



PL XIH, Fig. 3. 



Teeth small, concave in outline, lateral diameter somewhat less than 

 the antero-posterior diameter, thick. Convex crown-face low, gently 

 arched laterally, the convexity uniform with that of the basal area, 

 from which it is obscurely defiued by a faint coronal fold, or in worn 

 specimens by a shallow sulcus, gently arched downward in the middle 

 and again regularly curved to the obtuse lateral angles; the opposite 

 face presents a broad ovate outline, faintly convex laterally, with a 

 submedian depression in some instances, worn surfaces rather coarsely 

 punctate on approaching the crest, which is divided into three obtusely 

 acuminate lobes, of which the central one is a little the largest, the 

 lateral lobes being apparently of equal size. Basal area moderately 

 convex, inferior portion, as also the basal border of the concave crown- 

 face, unknown. The root probably resembles that of F. Mfidus, and the 

 concave coronal border was probably similarly bordered with imbrica- 

 ting folds. 



We have examined only two specimens of the form above indicated, 

 neither of which are entire, showing evidences of wear to which is 

 probably attributable the obtuse condition of the coronal cusps and the 

 absence of the imbricating folds, as also the obtuse, ill-defined lateral 

 angles of the crown. Tet these teeth present characters which unques- 

 tionably distinguish them from the previously described form. They 

 are remarkable for their robust proportions and tricuspidate crest, in 

 which respects they differ markedly from F. bijidus, and equally so from 

 the form of the allied genus, Cholodus incequalis, from the Upper Goal 

 Measures. 



Position and locality : Lower (?) fish-bed of the Chester limestone; 

 Chester, Illinois. 



GrEmrs CHOLODUS, St. J. and W. 



Teeth presenting the general outline and coronal contour of Peltodus, 

 but specially characterized by the division of the crest into two or more 

 eccentric, ina^qual cusps, of which the culminating cusp is lateral or 

 eccentric in position ; convex crown-face more or less regularly arched 

 with the basal area, broadly arched laterally, base defined by a faint 

 coronal fold, which is more or less eccentrically or obliquely arched 

 downward in the middle, and strongly deflected to the lateral margins, 

 terminating near the infero-lateral angles ; concave face more or less 



