10 GENBKAL ODSEUVATIUJsS. 



Associated with these, there are some other species that, if not identi- 

 cal with associates of those species just named elsewhere, are very closely 

 allied to them. Indeed, it is Avorth}^ of remark that there is so larg-e a pro- 

 portion of the sjiecies here referred to the Trenton period that are very 

 closely allied to well-known species, and yet present such minor differences 

 from typical forms, that I have felt obliged to append an inteiTOgation-point 

 to the names of the species I have refen-ed them to. The Graptolites prob- 

 ably belong to the Utica epoch of the Trenton period, and 1 have made 

 that provisional assignment of them. 



Two of the Nevada species, Grapfolithus ramulus White and G. hypni- 

 formis White are allied to two species, G. ramosns Hall and G. Whiffieldi 

 HalJ, that are found in the shales at Norman's Kill near Albany, N. Y., the 

 exact stratigraphical position of which shales has hitherto been in doubt. 

 One, G. ])ristis Hall! (not Hisinger), is apparently identical with a species 

 from the Utica slate of New York ; the other Nevada species is probably 

 identical with G. quadrimucronatus Hall, the type-specimens of which were 

 obtained from strata of the Utica epoch near Lake Saint John in Canada. 

 While the exact stratigraphical position of the shales at Norman's Kill has 

 not yet been demonstrated, the strata referred to at Lake Saint John are, 

 upon published evidence, refeiTed without hesitation to the Utica epoch. I 

 am not aware that any species found in strata of that epoch at the last- 

 named or at any other locality are identical with any found at Norman's 

 Kill ; but the relations of our Nevada species of Graptolites are very close 

 with some of those found at both of the eastern localities just mentioned. It 

 does not seem improbable, therefore, that this far western locality may be 

 found to furnish important evidence of the equivalency of the strata at 

 Norman's Kill with the Utica slate. (See note at end of chapter.) 



It is not unfrequently the case that Graptolites constitute the only 

 organic remains found in shales of Silurian age. This is doubtless due to 

 the fact that the physical conditions of the sea, in Avhich that kind of sedi- 

 mentary material which now constitutes the shale was deposited, were favor- 

 able to the existence of such forms of life, and at the same time made the 

 habitat an uncongenial one for other forms. The fact, therefore, that the 

 Belmont shales have furnished almost no other fossils besides Graptolites is 



