FOSSIL HEXACTINELLIDA 



207 



Fossil Hexactinellida. 



This group has the distinction of including among its 

 Lyssacine members the oldest 

 known sponge, Protospongia fenes- 

 trates, of Cambrian age (Salter). 

 As preserved it consists of a 

 single layer of quadriradiate, or 

 possibly quinqueradiate spicules, 

 which, arranged as a square 

 meshed lattice, supported the 

 superficial layer of the sponge 

 (Fig. 101). Whether or not the 

 fossil represents the whole of the 

 sponge-skeleton does not appear. 1 



The extraordinary Recepta- 

 culitidae are probably early 

 Lyssacine forms : they are cup- or 

 saucer-shaped fossils, abundant in Fig. 101. Part of the specimen of Pro- 



ci-i li ,, . -r. . tosponqia fenestrata iu the Sedgwick 



Silurian and above all m Devonian Museum, Cambridge. Nat. size. 



strata, and have been " assigned 

 in turn to pine cones, Foramini- 



( After Sollas.) 



and 



rm- 



fera, Sponges, Corals, Cystideans," 

 Tunicata. Hinde 2 brings forward 

 portant arguments for retaining them 

 among Hexactinellida. The only elements 

 in the skeleton of the simpler genera, e.g. 

 Ischadites, are structures comparable to 

 Hexactinellid spicules. The surface of 

 the fossil presents a series of lozenges 

 Fig. 102. a portion of the forming a regular mosaic. Each lozenge 

 is the expanded end of one of the rays 

 of a spicule ; it conceals four rays in one 

 panded outer rays of the plane tangential to the wall of the cup- 



spicules are partially de- r x 



stroyed, revealing the four shaped fossil, while the sixth ray pro- 



tangential^jays^beneath. j ectg vertically to the waU into the cav i tv 



of the cup. In the genus Peceptaculites 

 itself there is an inner layer of plates abutting against the inner 



outer surface of a Recepta- 

 culitid, Acanthoconia bar- 

 randei, in which the ex- 



1 Sollas, Quart. Journ. Geol. Soc. 1880, p. 362. 

 2 Quart. Journ. Geol. Soc. xl. 1881, p. 795. 



