VERTEBKATES. 81 



arched laterally, transversely rising into an obtuse and round- 

 ed mesial ridge; pores relatively coarse, those of the broader 

 slope elongated, more or less curved, but never inosculating, 

 the root small, half the height of crown, oblique on one side, 

 nearly straight on the other. Length, 11 lines; breadth, 4 

 lines ; height, 4 lines. 



The teeth of this species may be recognized by their smooth and rounded 

 outlines without points or angles, by the polished surface, coarse and elongated 

 pores. Iu their general aspect they bear a striking resemblance to the teeth of 

 Stroplwdus tanuis, Ag., (Poiss. Foss. Atlas, Vol. 3, tab. 18, fig 16-25), from the 

 Oolite of Stonesfield, though differing from these in microscopic structure. Of 

 described species of Helodus, they are most like some forms of H. Isevissimus, 

 but want any approach to a central cone. The resemblance which they exhibit 

 to the garden slug has suggested their specific name. 



Figures 5, 5 a, 5 b, represent one of these teeth seen from above, the side, 

 and the end, of the size of nature. 



Formation and locality: Burlington limestone, Burlington, Iowa. 



Helodus denticulatus, N. and W. 



PI. V, Fig. 6, 6 a, 6 b. 



Teeth of medium size, conical in profile, outline irregular, 

 middle portion broadest, ends rounded sharp, sides scolloped, 

 crown sloping evenly toward either extremity, from the most 

 prominent point, which is not central, strongly arched laterally, 

 bearing along the basal margin of one side numerous, conical, 

 tooth-like prominences, base showing faint traces of enameled 

 folds ; triturating surface uniformly, finely, punctate ; root as 

 high and broad as the crown, relatively thick, rough and 

 porous, oblique below. Length, 9 lines; height, 5 lines. 



The tooth will be readily recognized by the tooth-like appendages set along 

 the sides of the crown. 



Figures 6, 6 a, 6 b, side view, top and profile, natural size. 

 Formation and locality: Keokuk limestone, Hancock county, Illinois. 



—11 



