70 PALAEONTOLOGY OF ILLINOIS. 



its margin showed itself, and its affinity to the teeth upon which McCoy founded 

 the genus Pristicladodus, at once suggested itself. 



This species fully confirms McCoy's view of the classification of the teeth to 

 which lie gave this name, (Brit. Palatoz. Foss., p. G42.) With the one before 

 us there are now three species known, which, with striking departures from all 

 other types, have certain common characters, uniting them in a most natural 

 generic group. 



The name selected by McCoy for the genus also seems well chosen, for while 

 the broad, semi-circular, thickened bases of these teeth and their lateral denti- 

 cles, of which the exterior pair are largest, show their intimate relationship to 

 those of Cladodus, the broad, semi-conical, crenulated median cone, which 

 occupies nearly the entire breadth of the base, and so entirely overshadows the 

 lateral denticles, gives the group a facias quite distinct from that of Cladodus, 

 and show it to be a kind of connecting link between the Hybodonts and the 

 Carcharodonts. McCoy's name was, however, anticipated by perhaps the 

 equally appropriate one of Carcliarof>sis of Agassiz. 



The crenulation of the edge of the median cone in 0. Wortheni is remarka- 

 bly regular and distinct, and is continuous from the summit down to the lateral 

 denticles. The apex of the principal cone is somewhat worn, but was appa- 

 rently quite sharp. The anterior surface is not accessible, but is evidently 

 flattened, as in P. dentatus, McCoy, and somewhat excavated. 



Figures 14 and 14 a, posterior aspect and side-view — natural size. 



Formation and Joculity: Sub-carboniferous limestone, Huntsville, Alabama. 



Genus PETRODUS, McCoy. 

 Petrodus occidentalis, N. and W. 



PI. IV, Figs. 15, 15a, 155, 16, 1G a. 



Base sub-circular or elliptical in outline, flat or slightly 

 concave below, thin, terminating in an abruptly sharpened, 

 finely crenulated edge, broader than the crown ; crown broadly 

 conicaL acute or rounded at the summit, with a sub-circular or 

 elliptical section, constricted at the base, marked with a varia- 

 ble number of strong, divergent ridges, many of which are 

 forked below; ridges generally smooth, but sometimes more or 

 less rugose. 



