SILICIOUS AND HORNY SPONGES WILSON. 321 



proaches Isops. The same condition has been noted by Sollas for 

 S. nitida (Sollas, 1888, p. 231). 



The surface of the cloacal cavity is covered with oscular areas, 

 each including a single osculum (pi. 46, fig. 10; pi. 47, figs. 2. 3). 

 An oscular area, like a pore area, is a cortical region without 

 sterrasters but with abundant dermal spherasters. The areas are 

 rounded or irregular, 350 [/. to 1 mm. in longest diameter. Each area 

 is the depressed center of a low circular elevation of the general 

 cortex, the perioscular elevation (pi. 46, fig. 10, p. o. e.). The perios- 

 cular elevations are conspicuous to the eye, 1.5-2 mm. in diameter, 

 with some smaller ones, and 1-2 mm. apart. The open oscula reach 

 200 \K in diameter, but even these are doubtless partially closed. Many 

 oscula are completely closed. The osculum is the aperture of a 

 radial efferent chone-canal (r. e. <?.), the inner end of the chone, 

 (ch. e.), projecting into a subcortical cavity (crypt), (s. <?.). The 

 open osculum is surrounded by a dense aggregation of dermal 

 spherasters and these line the uppermost part of the chone-canal. 

 When the osculum and canal are closed, the spherasters mark the site 

 (pi. 46. fig. 10, o). The main part of the efferent chone-canal is 

 lined with oxyasters. The similarity of the oscular area as a whole 

 to a uniporal pore area is obvious. The difference is that the oscular 

 area is larger and is surrounded by the perioscular elevation. 



The skeleton includes closely set radial skeletal bundles made up of 

 orthotriaenes and oxeas. Throughout the interior of the sponge 

 there are abundant oxeas, scattered irregularly, separately and in 

 tracts. 



Spicules. — 1. Oxea. A stouter form 1250 by 30 p, and a slenderer 

 form 1000 by 14 \i, are both common in the radial bundles and in- 

 terior. Smaller sizes of the two types are abundant and the types 

 of course intergrade. The spicule is smooth, the points not very 

 sharp. 



2. Orthotriaene. Rhabdome about 1 mm. long, 35-40 [x thick, 

 tapering to a rounded point. Clads commonly about 220 ^ long, 

 24-28 [a thick at the base. But forms with shorter stouter clads, 

 about 170 by 35 pt, occur. Young stages plentiful. 



3. Sterraster, a somewhat flattened ellipsoid, 56 by 48 by 40 \k. 

 The sterraster crust is 230-280 p thick. Stages in development of 

 the spicule freely scattered through the interior. They show that 

 the development takes place in the usual way (Sollas, 1888, p. Lxrv). 



4. Spheraster (pi. 47, fig. 8, a), 6-8 \i in diameter. Rays numer- 

 ous but reduced to very low prickles, the centrum making up most 

 of the spicule. The rays are so short that with low powers the 

 spicule appears as a sphere. Abundant in dermal membrane; ex- 

 tending down into the pore and oscular canals, partly (fully half- 



