268 Evolution and Distribution of Fishes 



in considerable abundance, such as Spirifer mucronatus, 

 Cyrtia hamiltonensis, etc." As its fossils proclaim, we 

 would regard the Hamilton as a marine series. 



But elevation of land again took place, freshwater 

 lake deposits again formed, and these gave rise to rocks 

 of the Huron shales, that Newberry has described as "a 

 nearly homogeneous bituminous shale, attaining a maximum 

 thickness of 350 feet" {196: 13). Later he adds (p. 15) : 

 "The great fossil fishes found in the Huron shale have been 

 frequently referred to in the preceding volumes of this 

 report. Two species of Dinichthys, the largest and most 

 remarkable of all known ganoids," (now by many referred 

 to the Lung-fishes or Dipneusti) "one of Aspidichthys, and 

 one of Ctenacanthus are described and figured." And 

 again he says (197-191) that the Huron shale is "un- 

 doubtedly the source from which petroleum emanates in 

 such abundance in Western Pennsylvania." In Prosser's 

 more recent volume (p. 522) a calamite probably belonging 

 to the primitive genus Pseudobornia is also described. 

 Here in passing we would emphasize the intimate associa- 

 tion of petroleum with the giant fishes above named. 



But a new land depression caused re-invasion of the 

 sea, with resulting formation of the Erie shale, that may 

 be from 100 feet to 500 feet thick. This abounds in 

 "species of Spirifer, Orthis, Leiorhynchus etc." and so 

 is typically marine. But renewed elevation took place 

 (797:486) and the Cleveland shale was deposited in some 

 freshwater expanse. It is "usually a highly bituminous 

 shale containing 10 to 15 per cent of combustible matter." 

 As to its organisms he says that the fossils "are not numer- 

 ous or varied, yet it is not, as formerly supposed, entirely 

 unfossiliferous" for "at Newburg scarcely a fragment of 

 it can be found which does not contain scales of fishes" 

 while "the surfaces of the shale are also in this locality 

 sometimes covered with little comb-like fossils called by 

 Pander 'conodonts.' " 



With all of the above and related data before us it 

 is natural to find that Claypole gives the following neat 

 table : 



