VERTEBRATES. 61 



of the denticles; base small, 3 lines long, 2 J broad, lateral den- 

 ticles narrow, compressed and crenulated, generally unequal in 

 length and unlike in form; the anterior? one being longest, 

 divergent at a greater angle, long-pointed, acute, doubly-curved, 

 4-5 times as long as wide ; the posterior ? one straight or falcate 

 and relatively broader; the median denticle, when visible, 

 short, acute, compressed and crenulated. 



Figure 2, inner surface of large specimen, natural size. 

 Formation and locality : Coal Measures, Posey county, Indiana, and Linton, 

 Ohio. 



Diplodus duplicatus, N. and W. 



PL IV, Figs. 3, 3 a. 



Teeth small, slender; base relatively small and thin; denticles 



| four, sub-equal, conical, divergent, curved, somewhat striated 



longitudinally, with circular or sub-circular section, terminating 



in acute points ; entire altitude of tooth about 3 lines, length 



of denticles 2 lines, spread of the most divergent 4 lines. 



In the number of denticles this tooth differs so much from the species of 

 Diplodus heretofore described, that there may be some doubt whether it should 

 be included in the same genus with them. Its structure, however, is on the 

 whole much like that of D/gihbosus, D. gracillis, etc., the only striking differ- 

 ence being that this has four cornua, while they have two or three. The 

 number of denticles is, however, not constant in the genus — in the species I 

 have enumerated, the medial one being sometimes half the length of the two 

 lateral ones, sometimes reduced to a mere tubercle and sometimes even wholly 

 obsolete. Prof. Agassiz mentions having seen as many as five denticles attached 

 to one root, (Foiss. Foss., Vol. 3, p. 204.) 



In the figure of D. minutus {Atlas, Vol. 3, Fig. 7, Tab. 226) four denticles 

 are seen lying nearly in contact, and perhaps may have been connected with 

 the same root. Should it prove, however, that there were sometimes four den- 

 ticles on the tooth of that species, ours would still be distinguished from it by 

 its much more slender and awl-like cones. 



Figure 3 shows the posterior? aspect of an entire tooth, natural size; figure 

 3 a, the opposite face of the same, enlarged to two diameters. 



Formation and locality: Keokuk limestone, Nauvoo, Illinois. 



