DESCRIPTION OF GENERA AND SPECIES. 



CAMBRIAN SPONGES. 



Sub-Order. — Hexactinbllid^i. 



Family. — Protospongid^:. 



Genus. — Protospongia, Salter. 



1864. ' Quart. Journ. Greol. Soc.,' vol. xx, p. 238. 



Generic Characters. — Sponges probably cup- or vase-shaped, with walls consist- 

 ing apparently of a single layer of spicular mesh. This is composed of cruciform 

 spicules of varying dimensions ; the larger are arranged so as to form a regular 

 quadrate framework, which is divided into secondary squares by smaller spicules, 

 and these are again subdivided in a similar manner, so that, when complete, there are 

 four or five series of squares. The spicular rays appear to have been organically 

 cemented together at their points of junction with each other, and there are traces 

 of a delicate membrane in the interstitial areas between the rays, which may have 

 united the entire meshwork together. 



No other structures beyond the wall of spicular framework have as yet been 

 discovered, and it may be presumed that this constituted the entire skeleton of 

 the Sponge. 



This genus is, apparently, most nearly related to Dictyophyton, Hall, in which 

 the Sponge-wall is similarly constituted of larger and subordinate squares ; but 

 hitherto the spicular structure of these squares has not been described, and it is 

 quite possible that it may not have been of cruciform spicules like those of Proto- 

 spongia. From Phormosella, Hinde, the present genus is distinguished by the 

 regular arrangement of the larger and smaller squares of the meshwork, and from 

 Plectoderma, Hinde, by the simple, nonfasciculate disposition of the spicules. 



Mr. Salter defined the skeleton of this genus as " loosely reticular, formed of 

 very large cruciform spiculse, the branches of which cross each other at an angle 

 of 80°, and only in one plane, no ascending or descending branches rising from the 



