RELATIONS OF HELDERBERGIAN FAUNA 275 



Everywhere in the Cayugan, there is a scarcity of normal marine life, 

 and this is particularly true of the Salina and Rondout stages. The 

 Molluscoidea and Mollusca are generally absent. On the other hand, 

 the ostracods, phyllocarids, and particularly the eurypterids, are the 

 prevalent fossils. With the exception of the ostracods, not a single 

 species of phyllocarid or eurypterid is known in the Helderbergian in 

 any of the areas from Maine to the Indian Territory. 



With these facts there is presented a great paleontological break be- 

 tween the Siluric and Devonic at the top of the Cayugan group. The 

 succumbing of the normal marine faunas of the Niagaran group is 

 undoubtedly associated with the red gypsiferous and saliferous sedi- 

 ments of the Cayugan group. If the latter had a normal marine fauna 

 instead of one of peculiar Crustacea, the continuity of life from the 

 Niagaran to the Helderbergian would be probably complete. However, 

 in most areas outside of New York and Ohio, there is a great hiatus 

 between the Niagaran and the Helderbergian, which tends to make a 

 clear and easily discoverable line for field geologists in separating the 

 Siluric from the Lower Devonic. 



Relation to the Devonic* — In New York, where the Helderbergian is best 

 developed and its fauna well known, there are 364 described species, 

 with 33 new ones in the Beecher collection. Of this fauna more than 9 

 per cent go into the Oriskanian (this fauna has 17 per cent of Helder- 

 bergian species). It is therefore evident that specific identity is greater 

 between the Helderbergian and Oriskanian than between the former 

 and the Siluric. f 



Having examined the numerical specific relationships of the Helder- 

 bergian, its Siluric and Devonic aspects will now be pointed out. It is 

 needless to go into great detail at this time, as Clarke % did this work some 

 years ago. Therefore the more prominent features only will be treated. 



The trilobites are usually regarded as of first importance for strati- 

 graphic correlation. In the Helderbergian there are the following genera 



* The writer admits the unequal argument in contrasting the Helderbergian on one side with 

 the Niagaran and on the other with the Oriskanian. The Helderbergian and Oriskanian have 

 nearly continuous faunas, but between the former and the Niagaran intervenes the Cayugan group, 

 of which a meager fauna is known. However, the essential question is, Has the Helderbergian a 

 fauna more Devonic than Siluric in aspect? 



f The Siluric Manlius limestone has 26 species. Of these 3 are also in the Coeymans and 3 in 

 the New Scotland. 



The Coeymans limestone has 58 species. Of these 15 (2G per cent) are also in the New Scotland 

 beds, and 5 (or nearly 10 per cent) pass through into the Becraft limestone ; 4 (or 7 per cent) also 

 occur in the Oriskanian. 



The New Scotland has 298 species. Of these 15 come from the Coeymans, while 29 (or 10 per 

 cent) pass upward into the Becraft limestone, and 33 (or 11 per cent) also occur in the Oriskanian. 



The Becraft limestone has 44 species, of which 29 come from the New Scotland, and 12 (or 27 per 

 cent) pass into the Oriskanian above. 



\ The Hercynian Question, Forty-second Ann Kept. N. Y. State Mus., 1889. 



