﻿22 Bulletin 20 loo 



been used for the same stratigraphic horizon, the former name 

 having been used where limestone predominates over the shale. 

 It might be expected that where this horizon is represented by 

 limestone its fauna would resemble that of the Jefferson limestone 

 more decidedly than where a shale followed the Jefferson. 

 This presumption seems to be borne out by the small fauna 

 described from it by Girty. This shows that the total number 

 of species common to the Threeforks limestone fauna and the 

 Jefferson limestone fauna, as recorded in this paper, is only three 

 or the same number as the species common to the Jefferson lime- 

 stone, and the Threeforks shale at Threeforks. But the pro- 

 portion of the species common to the Jefferson limestone and the 

 higher horizon in the two cases is quite different, being 37 per 

 cent of the fauna described by Girty and only 4 per cent of that 

 listed by Raymond. The resemblance of the fauna of the Jef- 

 ferson limestone to that of the limestone fades of the Threeforks 

 formation is thus seen to be much more pronounced than its like- 

 ness to the fauna of the Threeforks shale. 



In attempting to determine just what part of the Devonian is 

 represented by the fauna of the Jefferson limestone we find that 

 two of the five species which are common to the Jefferson lime- 

 stone fauna and the eastern Devonian fauna are Atrypas. One 

 of these, A. reticularis, has no diagnostic value, while A. spinosa 

 has a recorded range from the Corniferous to the Chemung. 

 The other three species, Schuchertella chemungensis arctostriatus , 

 Productella spinulicosta, and Martinia maia, are known in the 

 eastern sections only in Middle Devonian horizons. The absence 

 from the fauna of any of the large Productellas, which generally 

 characterize the late Devonian faunas, together with the pre- 

 sence of an Upper Devonian fauna 25 following it in the section, 

 both supplement the intrinsic evidence of the fauna that, it is 

 earlier than Upper Devonian. The known range of the fossils 

 which are common to eastern sections suggest a Middle Devonian 

 age for at least a part of the Jefferson limestone. The fauna 

 contains no coarsely plicated Spirifers or other fossils generally 

 characteristic of the early Devonian, but other evidence strongly 



25 Am. Jour. Sci., vol. 23, 1907, pp. 1 16-122. 



