A Revision of the Sponges and Coelenterates of 

 the Lower Helderberg Group of New York. 



By GEORGE HERBERT GIRTY. 



Since 1861, when Prof. James Hall published volume 3 of the 

 Palaeontology of New York, no systematic review of the Lower 

 Helderberg fauna has been attempted. During that time the 

 fossils of this period have been much studied, not only in New 

 York but elsewhere, and many additions have been made both in 

 genera and species. 



Specimens from the Lower Pentamerus and Tentaculite beds 

 of the New York series are generally difficult to manipulate, on 

 account of the refractory nature of the very hard and sometimes 

 siliceous limestone which forms their matrix. On the other hand, 

 those from the Shaly limestone are usually somewhat crushed. 

 However, in certain layers of this horizon exposed at the Indian 

 Ladder, in the Helderberg raountain, the fossils occur very per- 

 fectly silicified, and at the same time the matrix is more 

 calcareous than is usually the case. This fortunate concurrence 

 renders it possible to etch the rock successfully, and specimens 

 obtained in this way compare not unfavorably with the products 

 of recent dredgings. Delicate fronds of Fenestella, arborescent 

 Bryozoans and spiny forms of Crania are not uncommon, 

 together with sponge spicules, annelid teeth, embryonic trilobites, 

 and other minute organisms which could not be obtained by 

 ordinary methods, since they would cot be observed in the field. 



The fauna itself is remarkable in many ways. In certain geo- 

 logical horizons, probably owing to contrasting physical conditions, 

 the different zoological groups were markedly localized. With the 

 Lower Helderberg Group it is different. The conditions seem to 

 have been congenial for the growth and preservation of nearly 

 all kinds of marine life at different periods during the deposition 

 of these strata, and in a single locality as many as 500 species are 

 known to occur, representing all, or nearly all, the fossil families 



