76 PALEONTOLOGY OF ILLINOIS. 



reticulation; root as broad and nearly as high as the crown, 

 very oblique, vertical face, marked with pits and vermicular 

 grooves, inferior surface roughened. 



This very distinct species is represented in the collection by a single frag- 

 ment only. That is, however, sufficient to show its general form and its struc- 

 ture. In form it is not unlike some species we have described, from the lower 

 rocks, but its microscopic structure is unlike that of any other species with 

 which we are familiar. The enamel tubes are much wider than the spaces 

 which divide them, and as they frequently coalesce, they form on the tritura- 

 ting surface a fine reticulation not unlike that of the surface of Strophodus. 



Figures 20, 20 a, represent side view and section, natural size. 



Formation and locality: Coal Measures, La Salle, Illinois. 



Helodus denshumani, N. and W. 



PI. IV, Pigs. 21, 21a, 216. 



Tooth small but strong, crown sub-quadrate in outline, the 

 angles being rounded, sides slightly arched, margins also 

 rounded, thicker on one side than the other, flattened above, 

 the central portion depressed on the anterior side, leaving an 

 elongated tubercle or prominence on either end, triturating sur- 

 face highly polished, marked with numerous, large, closely set 

 pores; root quadrangular, as wide as the crown and much 

 higher, coarse and long, sides and posterior face nearly straight; 

 the anterior face showing two concavities separated by a 

 prominent line, the upper much larger; largest diameter of 

 crown 4 lines; shortest 24 lines; entire height 5 lines. 



This peculiar tooth, which, taken by itself, naturally enough falls into the 

 genus Helodus, is probably one of the minor forms of some varied dental series. 

 Its robust figure and coarsely porous structure seem to indicate that it was 

 worn by a fish possessing considerable muscular power, perhaps of large size, 

 furnished with a strong crushing apparatus, composed mainly of large and 

 massive teeth, among which this was introduced to fill an interstice, or as one 

 of a medial series. Its form, not unlike that of a worn human bi-cuspid has 

 suggested the name given it. 



