92 PALAEONTOLOGY OF ILLINOIS. 



and allied genera when but slightly worn. Antero-posterior 

 diameter to extremity of prolonged angle, 1 inch, 8 lines; 

 transverse diameter, 1 inch, 8 lines; height, 11 lines. 



This fine tooth is not distinctly referable to any known genus, though com- 

 ing nearer to Cochliodus than any other. Waiting the discovery of other 

 specimens which will throw more light upon its affinities, we place it provision- 

 ally in Cochliodus. Associated as it was in the rocks from which it was 

 obtained, with the large species of Sandalodus, described by us under the name 

 of S. grandis — which it resembles in its massive strength and surface mark- 

 ings, we have thought it possible that it might have been a portion of the same 

 dental series with them. This is, however, mere conjecture, which will be 

 confirmed or disproved by future observation. 



Figures 2, 2 a, represent the upper surface and profile, natural size. 



Formation and locality : Keokuk limestone, Warsaw, Illinois. 



Genus ASPIDODUS, N. and W. 



Gen. Chae. — Teeth flattened, more or less arched on both 

 directions, variable in size and outline, sub-rhomboidal, oblong 

 or trapezoidal, the larger pair in each series having a trape- 

 zoidal outline, and being obliquely rolled, scroll-like, over the 

 jaw. The superior surface is smooth or nearly so, uniformly 

 and rather finely punctate throughout, the edges more or less 

 distinctly crenulated, the inferior surface smooth or slightly 

 roughened, corresponding to the jaw surface on which it rested. 

 The* smaller teeth are irregularly rounded or polygonic in out- 

 line, arched on both directions, resembling in form some 

 ancient shields. 



We have combined under the above generic title a group of teeth quite 

 largely represented in the collection, which seem to form a connecting link 

 between Cochliodus and Psammodus ; a portion of them having the twisted or 

 rolled form similar to that of the teeth of Cochliodus, others more nearly plain, 

 and approaching Psammodus in form and functions. They all want, however, 

 the ridges and furrows which characterize the teeth of Cochliodus, and differ 

 from the teeth of both the preceding genera in the crenulated edges. They 

 evidently formed a somewhat complex system of pavement-like dentition, which 

 covered an arched surface in the jaws of an ancient Cestraciont. 



