common to Carboniferous and Permian Strata. 207 



of Texas and New Mexico *. And it has lately been shown by 

 Dr. Prout that the Permian Polyzoan, Polypora biarmica, Key- 

 serling, is a Carboniferous species in the western states of the 

 same continent f. Aud there is little doubt, when the Permian 

 and Carboniferous species of this region become better known, 

 that other examples of recurrency will come to light. The ge- 

 neric relations of these life-groups are cei-tainly more intimate 

 in America than in Europe, as is evident from the occurrence 

 there of Orthocerus, Bellerophon, and Phillipsia% as Permian 

 types ; and it will be somewhat remarkable if their specific rela- 

 tions are not found to be as close. 



It is by such inquiries as the present that paleontologists will 

 ultimately be able to speak with more precision than they can 

 now on the amount of relationship that exists between the life- 

 groups of formations, and to arrive at juster estimates of the 

 relative value they possess as expressions of periods of geological 

 time. And though there is no reason to assume that we are in 

 a position to speak precisely in the present instance, it must yet 

 be evident that the relations that exist between the Carboniferous 

 and Permian fossils are most intimate — more so, apparently, 

 than usually obtain in the fossils of distinct systems of strata; 

 for this intercommunity of species is accompanied, as palaeonto- 

 logists are well aware, by a more than usual intercommunity of 

 genera, most of the common types of Permian strata being Car- 

 boniferous genera as well as Permian. This close alliance of 

 life-groups, viewed in conjunction with other evidence, caused 

 me, in a former paper, to question the propriety of our consider- 

 ing the Permian series of rocks a distinct system. It was there 

 pointed out that its importance as a stratigraphical group is 

 much less than that of other palaeozoic systems of strata ; and 

 that its life-phenomena, viewed either numerically in respect to 

 species, or generically in regard to the introduction of new types, 

 were of less consequence than those of systems, whether palaeo- 

 zoic or of later age§. Recent investigations have strengthened 

 this opinion. To use the term " system " in speaking of the 

 Permian group of rocks would seem to imply similarity of value 

 with other groups, such as the Silurian and Carboniferous, to 

 which the term more appropriately applies. It would apparently 

 indicate that it was of about equal importance in geological 

 classification, in the history of past life, and as the expression of 

 an interval of time, as either of the later groups, which I think 

 few geologists will be inclined to grant. It therefore seems 



* Trans. Acad. Science of St. Louis, vol. i. pp. 389, 391. 



t Ibid. p. 440. 



X Ibid. pp. 388, 399, 400. 



X Quart. Journ. Geol. Soc. vol, xvi. pp. 417, 419. 



