Lower Heldeeberg Fauna. 269 



Therefore to receive this and allied forms, I have proposed the 

 generic name Lysactjnella."^ 



The material on vrhich this geaus is established consists of 

 spioular casts in phosphatic nodules, isolated spicules, and an 

 entire sponge. The free spicules are preserved mostly as pyrite, 

 but, in a few examples, they are silicified. Two types of these 

 spicules are recognizable ; one extremely ornate, the other simple 

 and without spines. The complete specimen mentioned contains 

 only the simpler sort of spicules. In the large collection of 

 Lower Helderberg spoDges examined, no other entire specimen 

 of Lysaotinella was found. This must fairly represent the 

 various constituent spicular elements of the sponge, and probably 

 its original form. 



In shape it is a flattened sphere, and in this particular is 

 indistinguishable from the ordinary Hindia. A polished section, 

 however, clearly distinguishes the two types of structure, and 

 the regular radiating canals of Hindia are very characteristic. 



This genus will also include a number of Silurian hexactinellid 

 sponges, known only by scattered spicules, especially when there 

 is no reason to believe that they were attached by a basal tuft of 

 anchoring spicules, as in Pattersonia or Hyalonema. Among 

 such would be numbered Ulrioh's Hyalostelia solivaga mentioned 

 as isolated hexacts accompanying Hindia fibro8a.\ This 

 species may be identical with Lysactmella Gebhardi^ described 

 below. 



Sponge spherical to subspherical, sessile, i. e., without anchor- 

 ing spicules. Spicules (hexacts, pentacts, tetracts, etc.), without 

 such modifications as in Holasterella, obscure the systematic 

 arrangement and number of the spicular axes. 



Lysactixella Gebhardi, sp. nov. 

 Plate I, figures 1-31. 

 Sponge flattened-spherical, composed of hexacts, pentacts, etc. 

 Arms of the spicules simple rods, without ornamentation of 

 nodes or spines. 



As seen in section, this sponge appears as a mass of uncemented 

 spicules, so great in number that it is difficult to differentiate 

 individuals. These vary considerably in size and also in char- 



* Xbw^ I loose, and a-/yh^ spicule. 



tULRicH, 1890. Geol. Surv. Illinois, vol VIII, p. 2.32. 



