W. J. SOLLAS ON PHARETROSPONGIA STRAHANI. 245 



In using the term " poriferous " surface, we do not, of course, mean 

 thereby to imply that the intermeshes of the network of this surface 

 really represent the pores of the sponge, but simply that the surface 

 is now antoscular and was originally in close proximity with a pori- 

 ferous layer. The intermeshes, in all probability, represent the 

 openings of the incurrent canals that proceed from the " intermar- 

 ginal cavities " of the living sponge, of which intermarginal cavities 

 we seem to have indications in those minute furrows which we have 

 mentioned as giving a characteristic irregularity to the poriferous 

 surface. If this is the meaning of these furrows, they ought, according 

 to all analogy, to have been roofed over in the living sponge by a 

 delicate network for the support of the pores ; and that we do not 

 observe this now, is due to its disappearance after death, either by 

 decay or by attrition on the sea-floor. 



Such a superficial network would be connected with that which 

 originally formed the floor of the intermarginal cavities (and which 

 now lies exposed as the poriferous surface) by transverso fibres 

 passing from one to the other : the outer ends of these fibres would 

 disappear along with the network they supported ; but the internal 

 or basal ends might be expected to remain attached to the network 

 from which they spring. Of this nature appear to be certain conical 

 spines produced approximately at right angles from the network of 

 the poriferous surface, and contributing to its general asperity : it 

 is true these spines might possibly be the free ends of growing fibres 

 which were never attached to the outer network ; but since the sponge 

 grew marginally, and these are found on the wide surface of the plate, 

 this latter view seems less likely, and I think we may safely regard 

 the spines of the poriferous surface as the remains of fibres which 

 once supported a poriferous network which has now wholly disap- 

 peared, and the poriferous surface itself as being actually the floor 

 of intermarginal cavities, now unroofed and converted into pits and 

 furrows. 



Minute Structure, — For the investigation of the minute structure, 

 transparent slices were prepared by Mr. Cuttel, taken from the fossil 

 in two directions, one parallel to the surface of the sponge-plate, 

 and the other at right angles across it — parallel and transverse sec- 

 tions. The transverse section alone exhibits the distinction between 

 the primary and secondary fibres alluded to further on, as well as 

 traces of canalicular passages ; but as, with these exceptions, there 

 is no difference between the two sections worth mentioning, we 

 shall not require to speak separately of them again. 



The sections reveal the same irregular calcareous network which 

 is to be seen in roughly broken fragments. 



The primary fibres, or those which radiate towards the margins 

 of the plate in the direction of its growth, are not markedly different 

 in size from the secondary fibres which join them together, and 

 which, having formed conformably to successive growing edges of the 

 plate, naturally join them transversely. The distinction between the 

 primary and secondary fibres is most clearly seen in specimens from 

 the Upper Chalk from which the friable matrix surrounding and in- 



