354 PALAEONTOLOGY OF ILLINOIS. 



Genus HAKPACODUS, Agassiz. 



Teeth laterally elongated, vertically arched, and gently curved out- 

 ward in the concave face, with nearly parallel margins. Crown com- 

 pressed along the crest and deeply and regularly serrated, expanded 

 below, convex face relatively low, nearly vertical, opposite face gently 

 concave vertically, coronal borders more or less produced and sharply 

 inbeveled to the base in either side, laterally produced and sharply 

 rounded. Base strong, more or less obliquely produced to one or other 

 side, less the lateral diameter of the crown, lateral angles well-defined, 

 the relative depth of the two faces the opposite of the coronal faces, 

 that beneath the convex crown-face being deepest, gently excavated or 

 plane, the opposite side also laterally channeled, defined below by the 

 obtuse angle of the inferior surface, which latter is obliquely beveled to 

 the more acute posterior (?) margin. The coronal surfaces present the 

 usual dense, polished enamel-like external layer. 



Amongst some teeth from the Carboniferous limestone of Armagh, 

 belonging to the collection of Lord Ennisxillen, Prof. Agassiz recog- 

 nized a peculiar type to which he gave the above generic appellation, 

 and which, as originally understood, was supposed to be represented by 

 two distinct forms. One of these was designated by the specific term 

 H. dentatus and so published in the catalogue of types belonging to the 

 collection of Lord Ennisetllen, in 1S69 ; the other form, which was 

 represented by one or two imperfectly preserved specimens, had received 

 no specific designation. In the course of our investigations, we have 

 been so fortunate as to secure additional material, representing several 

 species which are closely allied to the forms noticed by Prof. Agassiz, 

 and from which may be derived a somewhat more satisfactory idea of 

 their character and relations than was afforded by the few European 

 examples accessible to him. 



Prom the study of these data, we have been led to transfer the form 

 H. dentatus to the genus Ctenopetalus, Agassiz, with wbich it apparently 

 has the most intimate relationship ; although strongly marked specifi- 

 cally by the extremely developed serrations: amongst the American 

 forms, however, we find those which, in the above respect, occupy an 

 intermediate position, thus more closely connecting this extreme form 

 with the more delicately serrated forms of the genus. 



Of the type represented by the unnamed form above referred to, our 

 collection also afford a few quite entire individuals, representing two 

 forms, which fully illustrate the distinctive and congeneric characteris- 

 tics wbich they in common possess with the European form alluded to 

 above, at the same time furnishing conclusive evidence as to their gen- 



