58 PALAEONTOLOGY. 



have been considered individually, either the various forms have 

 been interpreted as distinct species, or in some instances they have 

 been regarded as the representatives of distinct genera. This has 

 given rise to the establishment of generic groups which already \ve 

 see the necessity of abandoning, while the specific forms are largely 

 curtailed by their association into well-defined categories embracing 

 two, three, and even four distinct forms, all pertaining to a single 

 species. There can no longer remain a doubt as to the specific 

 identity of the four forms which Messrs. Newberry and Worthen 

 described under the head of Cochliodus nobilis (Cock, latus) of Dr. 

 Leidy ; and yet we are here furnished three groups formerly regarded 

 as possessing generic importance, viz: Cochliodus, Strcblodus, and 

 lldodus, in part. The announcement of Professor Eichard Owen of 

 the discovery of a third mandibular form of Cochliodus (the pre- 

 cise nature of which, however, we are not familiar with) very likely 

 will add a fifth form to those already noted under so-called generic 

 heads as really belonging to the genus Cochliodus. Very nearly the 

 same state of things obtains in relation to the genera Deltodus, 

 Sandalodus, Poecilodus, etc. Indeed the various forms of all these 

 Cochliodont genera have very generally received specific designations, 

 while some of them have been identified with genera widely differing 

 from one another, as is the case with the forms herein noticed under 

 the generic term Orthopleurodus. 



While the facts elicited by the stratigraphical knowledge accom- 

 panying .the greater part of the materials submitted to us have 

 rendered possible, indeed necessitated the revision of the genera of 

 the Cochliodonts, it has also developed interesting and important 

 facts bearing on the derivation and relations of these genera. That 

 these may be made comprehensible in briefest statement, the sub- 

 joined tabular review of the geological formations immediately con- 

 cerned, may not be inadmissible in this place. 



Table of Carboniferous formations, as developed in the region of 

 the Upper Mississippi: 



Upper Carboniferous \ E^SSSS 



( Chester limestone. 

 I St. Louis limestone. 

 I Warsaw limestone. 

 Lower Carboniferous'! Reokuk limestone. 



Upper Burlington limestone. 

 I Lower Burlington limestone. 

 LKinderhook beds. 



