92 PALEONTOLOGY OF ILLINOIS. 



Agass., of the Mountain limestone of Ireland ; and almost to the 

 same degree the maxillary posterior form named Streblodus Colei, 

 Agass., MSS., 1859, from the same deposits. The so-called "termi- 

 nai" tooth, which authors have associated with the first named 

 form, is strictly more intimately related to Paicilodus, and is homologous 

 with the mandibular posterior form, as shown by the alate postero- 

 lateral border, and although destitute of transverse undulations or 

 imbrications, it partakes in a marked degree of the coronal 

 conformation distinguishing the latter genus. Hence we are strongly 

 impressed with the conviction that the two forms heretofore associated 

 mder the designation D. acutus pertain to quite distinct generic 

 categories, and that there exists strong probability of the generic 

 identity with the present forms of the teeth referred to under the 

 term Streblodus Colei, and their possible specific identity with the 

 mandibular posterior (so-called "anterior") teeth of D. acutus. The 

 typical form of Streblodus Colei, however, presents distinctions which 

 might readily be regarded as emphasized specific variations, as 

 compared with the maxillary posterior form of Cochliodus contortus, 

 Agass., (Streblodus oblongus, Agass.) ; and at the same time it pre- 

 sents an extreme departure from the normal characters distinguishing 

 the same form which is here ascribed to representative American 

 species of Deltoptychius. From the latter the Irish form is distin- 

 guished by the considerable breadth of the depressed interval 

 separating the narrow secondary lobe from the posterior coronal 

 prominence and the consequent relatively narrower anterior neck, 

 which latter is nearly plain and terminated precisely as above de- 

 scribed. It will, therefore, be observed on close inspection of the 

 Irish and American forms that, while they differ in the above 

 respect, they are more intimately related to one another than the 

 former is with the maxillary posterior tooth of Coch. contortus (Streb. 

 oblongus,) and it is on these grounds we have been led to suggest 

 its generic identity with Deltoptychius, as herein amended. The 

 Irish form referred by authors to the "terminal" tooth of Deltopty- 

 chius, is not met with in this country, unless, as has already been 

 mentioned, it should be regarded as identical with the mandibular 

 form of Poicilodus. We are, however, not aware that the Irish local- 

 ities have afforded other Poicilodus forms which might be specifically 

 identified with the latter. 



