SILICIOUS AND HORNY SPONGES WILSON. 381 



considerable part of the surface, and on one side the body has been 

 torn badly. 



In the equatorial zone are deep poriferous pits, with smooth lining 

 membrane and without marginal fringes. The lining membrane 

 shows small closely set pore areas separated by narrow trabeculae. 

 The cavities underlying the membrane are no larger than the sub- 

 dermal cavities of the general surface, a fact which tends to indicate 

 that the pits are afferent. In the specimen there are two perfect pits 

 and parts of two others. The largest has an equatorial width of 14 

 mm., the others about half that. 



A small, very eccentrically placed osculum, about 1 mm. in diam- 

 eter, in a depressed smooth area, is present on the upper surface. 

 Perhaps other oscula were present in the torn region. 



The ectosome is not distinctly fibrous. There is a fairly extensive 

 system of subdermal cavities, from which narrow canals pass into 

 the interior, and into which small pore canals pass everywhere from 

 the dermal surface. The abundance of sigmas in the walls aids in 

 tracing the pore canals. 



The under surface was doubtless attached in places and was torn 

 loose from its substratum. Nevertheless the dermal membrane over 

 a large part of it is uninjured. This surface is very different 

 from the rest, which is much incrusted with sand grains, etc., and 

 is hispid with the projecting spicules of the radial skeletal bundles. 

 The under surface, on the contrary, has a smooth, clean, thin dermal 

 membrane showing small pore-areas separated from one another by 

 a net-work of narrow trabeculae. It is quite similar to the membrane 

 lining the poriferous pits. 



The skeletal bundles pass radially outwards to all parts of the 

 sponge from about the center of the lower surface, the lowermost 

 bundles lying lengthwise at this surface in plain view. Doubtless 

 the detailed character of the lower surface varies in accordance with 

 the local conditions affecting the individual. 



Spicules (pi. 48, fig. 6). — 1. Oxea, equiended, 3.5 mm. by 50 [x, 

 with smaller sizes. The chief spicule of the radial bundles, the more 

 superficial ones projecting from the surface. 



2. Oxea, small and slender, about 360 by 2-3 [jl; scattered, not very 

 abundantly, between the skeletal bundles. 



3. Orthotriaene with short rhabdome; the characteristic ectosomal 

 spicule ; abundant. Typically the rhabdome is radial to the surface, 

 the clads tangential ; the spicules forming one to about three layers. 

 In a typical spicule the rhabdome measures 120 [x, the clads 370 jx in 

 length; larger and smaller sizes occur; rhabdome in general about 

 one-third the length of a clad. Rhabdome straight, tapering to a 

 point, or more cylindrical. Clads with the usual bowlike curvature; 

 those of a spicule generally alike. 



