57 ' 



their entire length, in close connection with each other , so 

 that the rounded projections on their surfaces are, as it were 

 morticed together. The not infrequent specimens in which the 

 spicules yet remain attached together, prove that the skeletal 

 mesh was better able to resist disintegrating influences than 

 most of these Chalk Lithistids. The surface or dermal spi- 

 cules of this sponge strikingly differ from those of most other 

 Lithistids. They consist of extremely v thin delicate plates or 

 laminae, of regular and irregular . forms, which appear to have 

 formed a covering over the exterior of the sponge, their edges 

 overlapping each other. There is no trace of any shaft as in 

 the surface discs of Discodermia, and these spicules must have 

 been held in place by the living sarcode. Neither is there 

 any indication of interior canals; so that, in this instance at 

 least, the surface spicules cannot be regarded as modifications 

 of the skeletal mesh spicules. Some of these dermal spicules 

 have the form of straight laminae, widest in the centre and 

 gradually tapering to either end. Others of about the same 

 width are curved, and the lower portion bent round in such 

 a manner that the spicule has a tendency to stand on edge, 

 as shown in fig. 44. Other spicules are circular or oval in 

 outline whilst some few are irregular in form. Of each re- 

 gular form of these laminated spicules there are numerous 

 examples in the Horstead flint, those of each form agreeing 

 also in size with each other. The relations of these diffe- 

 rently shaped laminae to . each other was apparent at the time 

 of my picking them out of the deposit , but had they not 

 been discovered in their natural position on the surface of the 

 sponge with the well-marked mesh spicules, I think it would 

 have been inpossible to have recognized in them the dermal 

 spicules of a Lithistid sponge. 



The sponges to which these mesh spicules and laminated 

 dermal spicules belong, Plinthosella squamosa Zittel, are small 

 spherical bodies from 5 mm. to 25 mm. in diameter. The 

 spicules form a coarse irregular mesh work, traversed by canals 



