108 BRITISH PALAEOZOIC SPONGES. 



spicular mesh is composed of robust cruciform spicules, the rays are approximately- 

 rectangular, and nearly of a uniform thickness throughout their length. The 

 centres of the spicules are slightly elevated, so that they are not strictly horizontal. 

 The rays of the smaller spicules in the majority of cases dip beneath those of the 

 larger forms. Five series of squares are present in the complete mesh, the largest 

 are 8 mm. in diameter and the smallest '5 mm. ; the axes of the largest spicules 

 are 16 mm. in length and "52 mm. in thickness, whilst the smallest are 1 mm. in 

 length and "2 mm. in thickness. 



The typical example of this species, now in the Woodwardian Museum at 

 Cambridge, is preserved on a block of black slate. The spicular mesh has been 

 replaced by iron-pyrites ; in places it stands boldly out from the rock surface. 

 Though the regular arrangement of the spicules of the mesh is clearly shown in 

 only one portion of the specimen, it can be traced over an extended surface, and it 

 occurs at two different levels separated by an interval of matrix, of about 4 mm. in 

 thickness. This appears to me to indicate that the entire Sponge was vasiform or 

 cup-shaped, and that, owing to pressure, the opposite walls of the cup are now 

 nearly in contact with each other. 



The original specimen was discovered by Dr. H. Hicks, F.R.S., who referred it 

 to P. fenestrata, Salter. It was subsequently described in considerable detail by 

 Prof. Sollas, who also regarded it as identical with Salter's species. A comparison 

 of this form with the type of P. fenestrata shows, however, a very considerable 

 difference in the thickness of the spicular rays, sufficient to indicate it as a distinct 

 species, which I have named in honour of its discoverer. 



In no case in this specimen are the points of contact of the spicules with each 

 other clearly shown, but the structure of the mesh appears to me to justify the 

 view that the spicules are cemented together where they join each other ; Prof. 

 Sollas states, however, that they are separated and not united either by envelop- 

 ment in a common coating or by ankylosis. 



Fragments of mesh and detached cruciform spicules, apparently belonging to 

 this species, have been discovered in Norway, Sweden, and also in Nevada, at 

 approximately the same geological horizon. 



Distribution. — Cambrian ; Menevian Group. Porth-y-Rhaw, near St. David's, 

 South Wales. Cambrian ; Paradoxides- Shales, Krekling, Norway (Brogger) ; at 

 Andrarum, Sweden, in beds with Paradoxides and Agnostus pisiformis ; Eureka 

 district, Nevada, in the Prospect Mountain Group (Walcott). 



