60 PALAEONTOLOGY OF ILLINOIS. 



Among the specimens which I have been inclined to refer to this species, 

 considerable diversity of size and form may be noticed, and it is. by no means 

 certain that they should be regarded as specifically identical. Of these the 

 largest constitute the true type of the species, as first described. They are 

 characterized by the great strength of the lateral denticles, which are scarcely 

 twice larger- than wide at base, and nearly of equal length, accurately set in 

 the same vertical plane. Another group, little smaller than these, are however 

 sensibly more slender. The lateral denticles, though as long, are narrower and 

 scarcely compressed at base. Ihe anterior tubercle is much more prominent, 

 and the posterior portion of the base has a vertical groove in the edge, at the 

 mesial line, and above this a broad, flattened, circular tubercle, which is rough- 

 ened on the upper surface, as though it was the place of a muscular attachment. 

 A similar character is shown in the magnified figure of D. gibbosus, Ag. (Poiss. 

 Poss. Atlas, Vol. Ill, Tab. 22b, Jig. 4), but there the flattened tubercle is rep- 

 resented as though placed in apposition with and between the bases of the 

 lateral denticles. In all the specimens before me it is separated from the cusps 

 by a space equal to its own diameter, which is about 2 lines. In these two 

 groups, which I have hardly been willing to separate specifically, the centra] 

 cusp is either entirely obsolete or reduced to a small conical tubercle. 



Another group of teeth, which are not rare in the Coal Measures of Ohio, is 

 represented in the collection by two specimens only. These I have formerly 

 designated by the name of D. compressus, though it is possible they should be 

 regarded as merely a variety of D. latus. They are, however, not more than 

 half the size of the teeth of that species. The base is relatively smaller, the 

 cornua more unequal, acute and compressed; one of the pair — the right or left, 

 according to the position of the tooth in the mouth — being much narrowed 

 toward the point and sigmoidally curved. In this group the margins of the 

 denticles are more distinctly crenulated. (See description of D. compressus.') 



Figures 1 and 1 e, side views; la, lb and 1 c, anterior faces of different speci- 

 mens; Id, posterior face — all natural size. 



Formation and locality: Coal Measures, Posey county, Indiana, and Colum- 

 biana county, Ohio. 



Diplodus compressus, Newb. 



PI. IV, Fig. 2. 

 Diplodus compressus, Newb., loc. cit. 



Teeth smaller and more slender than those of D. latus; about 

 5-6 lines high, and having a space of 4 lines between the points 



