SILICIOUS AND HORNY SPONGES WILSON. 281 



few of the large skeletal oxeas intermingled. Round the margin of 

 the cloaca! aperture (pi. 45, fig. -A, c. m.) these spicules, here some- 

 what longer, are inclined obliquely to the upper surface and radiate 

 toward the center of the aperture, constituting a cloacal fringe. 

 Much of the fringe has been destroyed, but in places it projects 4-5 

 mm. 



The outer margin (pi. 45, fig. 4, o. m.) of the upper surface is a 

 sharp edge which distinctly separates this surface from the latero- 

 inferior surface of the sponge. At this edge the protruding spic- 

 ules of the upper surface project radially outward and are longer 

 than elsewhere, projecting in places 5-6 mm., constituting a marginal 

 fringe which has been in large part destroyed. 



The latero-inferior surface (pi. 45, fig. 4, I. .<?.) of the sponge is con- 

 siderably injured. Nevertheless enough remains to show its charac- 

 ter. It is smooth and is riddled with evenly scattered pores about 

 50 [x in diameter. Here and there long slender oxeas protrude. 



The wall of the cloaca (pi. 45, fig. 4, c. id.) is smooth; not uni- 

 formly lined with a fenestrated membrane. It is largely occupied 

 by the open apertures of canals 3-5 mm. in diameter, between which 

 are the mouths of some smaller canals closed in with porous mem- 

 branes. The sponge is fragile ; excavated by numerous canals, many 

 as large as 3-5 mm. in diameter. 



The upper surface of the sponge is supported by the tangentially 

 placed clads of abundant triaenes. These are absent over the latero- 

 inferior surface, but a few are found in the interior of the sponge. 

 The latero-inferior surface is supported by the ends of radially 

 placed large oxeas (the strongyle variant is not infrequent), usually 

 grouped in bundles of a few spicules. Through the interior the 

 large skeletal and the long slender oxeas are freely scattered. The 

 parenchyma is further supported by the microxeas with which it is 

 densely filled. 



Spicules. — 1. Orthotriaene with short rhabdome. Clads 500- 

 1100 by 50-80 y., characteristic size about 900 by 60 [jl; clads of a 

 spicule frequently unequal in length. Rhabdome generally one-third 

 to one-half the length of a clad, occasionally as long as a clad; 

 commonly 250-350 by 50-70 \i. 



2. Oxea, large skeletal form. Smooth, slightly curved, tapering 

 evenly to points ; 2.5-4.5 mm. by 00-90 p, with smaller sizes down to 

 1800 [i. by 35 \i. The large sizes are characteristic. The spicule is 

 sometimes strongylate. 



3. Oxea, slender form; 1.5-12 mm. by 10-20 [a; common lengths 

 2-7 mm. The spicule is occasionally strongylate. 



4. Small oxea (microxea), about 250 \i by 4 p.. 



