VERTEBRATES. 79 



obliquely conical profile it approaches more nearly to the teeth of some living 

 sharks, such as Galeus, Galeocerdo, etc., than any of the forms with which it is 

 associated. 



Figure 1 is a front view, twice the natural size. 



Formation and locality: Burlington limestone, Augusta, Iowa. 



Helodus politus, N. and W. 



PI. V, Fig. 2. 



Teeth small, laterally elongated, outline linear, expanded at 

 the centre, somewhat curved, profile strongly arched or rudely 

 conical ; crown composed of a relatively large central gibbosity, 

 from which slender appendages are given off, forming the lat- 

 eral extremities ; both the central and lateral portions strongly 

 arched transversely, so as to constrict the crown below; entire 

 crown surface covered with dense, homogeneous, black, polished 

 and shining enamel, in which no pores are distinctly visible; 

 root small, oblique, having generally disappeared or remaining 

 only as an ochery mass; length, 7 lines; height of crown, 2 lines. 



This peculiar tooth is not likely to be confounded with any which we have 

 described. It will be at once recognized by its singular form and by its highly 

 polished and poreless surface. Specimens of H. gibbemdas, Ag., now before us, 

 from the Armagh limestone, approach closely to it in form, but in that species 

 is very distinctly, even coarsely porous. 



Figure 2 represents the posterior aspect, natural size. 



Formation and locality : Keokuk limestone, Nauvoo, Illinois. 



Helodus gibbosus, N. and W. 



PI. V, Pigs. 3, 3a. 



Teeth of medium size ; outline as seen from above, linear, 

 with a central protuberance on one side ; extremities rounded, 

 profile conical, summit of cone nearest one end, crown strongly 

 arched transversely, and slightly constricted at its base, surface 

 uniformly and finely porous. On one side a conspicuous conical 



