VERTEBRATES. 75 



ble number of the species already described sbould be carefully compared with 

 those now before us, and it is perhaps wiser to wait the time when this shall be 

 possible, rather than to attempt now to make the sub-divisions to which we 

 have referred. It seems "to us not probable that Helodus should be regarded, as 

 suggested by Prof. McCoy, as a sub-genus of Psammodus, or at least that any 

 other than the last group referred to should be so considered. The flat, quad- 

 rangular teeth of Psammodus form a very natural generic group, and the 

 porosity of their surfaces is no more like that of Helodus than Cochliodus. 



Helodus coniculus, N. and W. 



PI. IV, Figs. 19, 19 a. 



Teeth small, laterally short, crown composed of a relatively 

 high, rounded central cone, with short lateral appendages; 

 these latter are rounded or obliquely truncated, and project 

 not quite half the diameter of the cone from its base. A sulcus 

 surrounds the crown constricting it, at its junction with the 

 root; root not very short, nearly as broad as the crown, 

 oblique, its vertical face deeply impressed with vermicular cavi- 

 ties; the crown surface is smooth and polished, and uniformly 

 porous throughout. 



This species is closely allied to H. didgmiis, Ag., but seems to be distinguish- 

 ed from it by its relatively larger cone, the sulcus around the crown, and the 

 pitted root. 



Figures 19, 19 a, give a front and side view, natural size. 



Formation and locality: Burlington limestone, Burlington, Iowa, and Keokuk 

 limestone, Nauvoo, Illinois. 



Helodus carbonarius, N. and W. 



PI. IV, Figs. 20, 20 a. 



Teeth small, transversely elongate, crown linear in outline, 

 gently arched laterally, strongly so transversely, smooth and 

 polished throughout, marked with numerous relatively coarse 

 pores, which frequently coalesce, forming a fine but irregular 



