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 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 cach 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, Plxthosella 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 
