288 PALEONTOLOGY OF ILLINOIS. 



that of the inner face is smooth in all our specimens, though in well- 

 preserved individuals it will probably be found to be similarly orna- 

 mented. The base presents all the characteristics of the above group 

 of teeth, the anterior marginal ridge is well defined, nearly straight, 

 and but slightly arched vertically; inferior surface moderately exca- 

 vated, the posteriorly expanded portion projects sharply downward at 

 a slight angle to the posterior crown face, broadly rounded along the 

 inner margin. Lateral diameter of crown about .35 inch, or slightly 

 exceeding that of the base; elevation of outer face of tooth .16, greatest 

 thickness of crown at base .07, anteroposterior diameter of base .10 

 inch. 



The original of the present form, although somewhat abraded in the 

 coronal portion, and so imbedded as to expose only the outer aspect of 

 the tooth, is readily identified with more perfect specimens since obtained 

 from the same horizon at other localities, which latter teeth exhibit, 

 apparently, unquestionable generic relationship with Jlybocladodus, and 

 dissimilar in all essential characters from Helodus, as evidenced by the 

 peculiar form of the base and acuminate crest. The compressed crown 

 and laterally deflected median cone afford the widest contrast with the 

 typical species Hybo. plicatilis, with which it is associated, but which 

 more intimately ally it with Hybo. nitidus, described from the Chester 

 limestone, from which latter, however, it is distinguished by its more 

 compressed crown, and, perhaps, greater regularity of the crest line, 

 etc. 



Position and locality : Fish-bed of the Upper Burlington limestone ; 

 Louisa county, and Augusta, Iowa. 



Htbocladodus nitidus, St. J. and W. 



PI V, Kg. 7. 



Teeth very small. Outline of base elliptical, considerably expanded 

 posteriorly in the same horizontal plane as the crown, and gently arched 

 vertically, moderately excavated below, anterior marginal ridge strong 

 and well defined from the crown, slightly produced outward. Crown 

 strong, prominently distended in front, defined from the base behind 

 by a slight angle, constricted at the extremities, crust obtusely angular, 

 median prominence eccentric, strong, and laterally deflected, with a 

 small, obtuse secondary cone at either extremity, surfaces apparently 

 destitute of ornamentation. 



This species is founded upon a single specimen, a nearly perfect tooth, 

 which measures scarcely one-tenth of an inch in lateral diameter, and 

 is the sole representative of the genus thus far brought to our knowledge 

 in the upper division of the Lower Carboniferous deposits. Closely 



