VERTEBRATES. 27 



Nob., approaches it nearest, but while the median cone of that species is alike 

 conical and similarly striated, the lateral denticles ar,e more numerous, and the 

 thicker base, with its beveled anterior margin, roughened with a thousand 

 minute spines, furnishes a 'character which will serve to distinguish it at a 

 glance. 



Figure 11, anterior face of broken tooth, with section of median cone; 11 a, 

 base underside, natural size. 



Formation and locality: St. Louis limestone, St. Louis, Missouri. 



Cladodtjs politus, N. and "W. 



PI. I, Fig. 12. 



Teeth small, as high as broad; base flat below, nearly 

 straight before, terminating in a somewhat acute edge poste- 

 riorly, and on either side; median cone conical, very acute, 

 slightly compressed, without cutting edges, smooth and polish- 

 ed throughout; lateral denticles one on either side, with an 

 inner rudimentary one, scarcely more than a tubucle. 



The anterior face of C. lamnoides is nearly as smooth and highly polished, 

 and, when not fully exposed, these species might be mistaken for each other; 

 but in G. politus the lateral cusps are not set on the edge of the base, are diver- 

 gent, and a rudimentary denticle is usually visible between the exterior one 

 and the central cone. 



There is in the collection another perfectly smooth tooth which is scarcely 

 distinguishable from the specimens of C. politus, except that it has cutting- 

 edges. It may be merely a variety, and at least is too near like it to justify us 

 in making a distinct species out of it without more material. That tooth was 

 obtained at Honey creek, in Henderson county, from a different geological 

 horizon, and very possibly will prove distinct. 



Figure 12, anterior aspect, natural size. 



Formation and locality: Chester limestone, Chester, Illinois. 



Cladodus costatus, N. and W. 



PI. I, Figs. 13, 13 a. 



Teeth small ; base flat, thin, anterior border straight, or 

 slightly convex, beveled upward and backward to the bases of 



