VERTEBRATES. 59 



In its general aspect this tooth is much like the last described. Like that, 

 its disproportionate breadth renders it very unlike the teeth of any of the true 

 sharks of which we have any knowledge. The Rays and some Cestracionts 

 have equally broad teeth, but these all have flat or obtuse summits, fitted for 

 grinding or crushing; none of the incisive teeth of the carnivorous sharks hav- 

 ing anything like an equal relative breadth. It would be a matter of great 

 interest to ascertain, if it were possible, what was the position of these broad, 

 knife-like cutting teeth in the mouths of their possessors; whether they were 

 set transversely across the symphysis of the jaws, as in the Myliobatini, or on 

 the rami of either side, as in the Cestracionini. 



Their perfect symmetry would rather indicate a medial position like the broad 

 teeth of the rays. Whether in this case they formed a single series, as in 

 JStobatis, or were associated with narrower lateral teeth, as in Zygobatis and 

 Myliobates, we may perhaps never know, as the cartilaginous jaws on which 

 they were set have usually been wholly decomposed and the teeth scattered. 



Figures 19" and 19 a represent the face and profile, natural size. 



Formation and locality : Keokuk limestone, Warsaw, Illinois. 



Genus DIPLODUS, Ag. 

 ' Diplodus lattjs, Newb. 



PI. IV, Pigs. 1, le. 

 Diplodus latus, Newb., Pise. Phil. Acad. Nat. Sci., 1856, p. 99. 



Teeth large and robust; base thick and massive, elliptical 

 in outline, flattened below, with a large, obtuse tubercle pro- 

 jecting downward and forward at the anterior margin; posterior 

 end rounded; central cusp rudimentary or obsolete; lateral 

 denticles unequally divergent in the same plane, very robust, 

 2-3 times* as high as broad at base, smooth and polished 

 throughout, compressed, with a lenticular section, and sharp, 

 crenulated, cutting edges, extremities sharp, but scarcely 

 pointed. 



This is a large and robust species, the larger specimens being 9 lines high; 

 the extremities of the lateral cusps being separated by an equal interval. Un- 

 like D. gibbosus, Ag., and D. gracilis, Newb., the lateral denticles diverge in 

 the same plane, which is vertical to the antero-posterior axis of the base. 



