Lower Helderbeeg Fauna. 267 



framework was probably overlaid with pyrite while it was still 

 in a perfect condition. The spicules themselves were then dis- 

 solved out, and a deposition of pyrite over the exterior sealed up 

 the structure from further alteration. An oxidization of the 

 pyrite at the center to limonite finally reduced the specimen to 

 the condition in which it is now presented. 



The evidence afforded by the Lower Ilelderberg specimens as 

 to the original composition of Hindia is anything but conclusive, 

 and at first sight seems to contradict the siliceous nature of the 

 sponge. Specimens of Lysactinella, the undoubted Hexactinel- 

 lid genus described below, are represented in the majority of 

 cases by amorphous pyrite, while Hindia, as before stated, is never 

 so preserved. On the other hand, a few examples of Lysactin- 

 ella are replaced by crystalline pyrite, and no calcareous tests 

 occur pyritized. Thus, while Hindia differs in preservation 

 from, most examples of a known siliceous organism, it is also un- 

 like the brachiopods and other calcareous forms from the same 

 beds which are usually silicified. In view of its structure, which 

 is clearly Lithistidan in type, and of the facts adduced by Hinds 

 and RiuFF, despite its ambiguous preservation-characters, the 

 preponderance of evidence is plainly in favor of the position 

 taken by those writers, that Hindia is a siliceous sponge of the 

 order Lhhistida. 



The smallest specimen observed by me in the collection is 6 

 mm. and the largest Gi mm. in diameter. This shows a much 

 greater range in size than noticed by Hinde, whose measurements 

 are 13 to 45 mm., or by E-auff who gives 10 to 45 mm. 



Formation and locality. — Lower Pentamerus and Shaly lime- 

 stones of the Lower Helderberg group, at Clarksville, Indian 

 Ladder, and other localities in New York. The same species is 

 cited from the Silurian of Russia, Scotland, Germany, JSTew 

 Brunswick, Indiana, Kentucky, western Tennessee and Minnesota. 



Order HEXACTINELLIDA. 



Suborder LYSSACINA. 



Lysactinella, gen. nov. 



The presence in the Lower Helderberg Group of a hexactinellid 

 sponge, belonging to the Lyssacine order, was first made known 



