THE SPONGES. 137 



gradually toward the edge which is rounded and projects outward. The 

 lateral surface of the sponge is divisible into six uneven faces, which slope 

 suddenly away from the upper edge towai'd the contracted base. The 

 height of the mass is 50 ram., its greatest width 75 mm. Behind the main 

 body, when the latter is seen in the position of Fig. 7, Plate 18, the 

 sponge extends for a short distance in incrusting fashion over the con- 

 glomerate, and then rises up in the shape of a small nearly vertical lamella 

 which is partially divided into two lobes and is about 5 mm. thick. 



The upper surface of the main body is covered with a smooth dense 

 membrane, which is quite imperforate save for the oscula. A similar mem- 

 brane covers the ridges and prominent parts of the lateral surface and one 

 surface of the lamellate continuation. The more depressed parts of the 

 lateral surface of the main body, comprising the greater part of this surface, 

 appear to the eye rough and comparatively porous, and one surface of the 

 lamellate continuation has this appearance. Microscopic examination shows 

 that in the rough regions the dermal membrane is plentifully pei'forated 

 with pores. Thus the surface of the sponge is differentiated into pore and 

 oscular regions, these regions occupying opposite surfaces where the body is 

 lamellate, but intermingling to some extent where the body is massive. 



The oscula measure 1 to 2 mm. in diameter, and are found scattered 

 irregularly over the smooth regions, sometimes singly, but more often in 

 small groups of two to four. In the rough regions numerous pore mem- 

 branes roofing in canals are distributed irregularly. The membranes are 

 rounded or irregularly shaped, perforated by one to a few j^ores, and 

 measure from 2 to 3 mm. to a fraction of 1 mm. in diameter. The indi- 

 vidual pores are mostly about 200 /a in diameter, with larger ones occurring 

 less frequently. 



The color of the surface is white, that of the interior light brown. The 



sponge is firm. The flagellated chambers are rounded, 28 to 36 /x in 



diameter. The arrancrement of the chambers and canals indicates that the 

 chambers are eurypylous, although the actual openings cannot be made out. 



Spicules. Mecjascleres. 1. Tylostyle, Fig. 5, a~d, Plate 18; smooth, 

 sharp-pointed, slightly curved ; head well marked ; tapering toward head 

 end as well as toward point. Size varies from 1275 /a x 36 /x to 290 /x x 8/i. 

 Rarely the spicule appears strictly diactinal, bearing an enlargement at some 

 point along its course, whence it tapers to a point at each end. 



Microsclires. 2. Sigmata, Fig. 6, a-(j, Plate 18, scantily distributed 



i» 



