408 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



PETROSIA CRUSTATA, new species. 



Plate 41, fig. 6; plate 49, fig. 2. 



Station D5414, one specimen. Sponge an amorphous mass, thin 

 and incrusting below, that has grown round and is penetrated by a 

 Gorgonia-like alcyonarian. Greatest diameter 60 mm. The sponge 

 is surmounted by a Discodermia emarginata (the compact mass at 

 the summit in pi. 41, fig. 6), and is infested with small barnacles, 

 round each of which the sponge tissue forms an elevation open at 

 the summit; the apertures, scattered over the surface, look at first 

 sight like oscula. 



Surface of sponge white, smooth, aporous (to eye and lens), and 

 stony. Interior whitish with a tinge of yellow, firm but far from 

 ston\ T , friable; permeated by numerous canals visible to the eye, 

 these varying in diameter from minute ones up to canals 2 mm. 

 wide, but comparatively few are over 1 mm. wide; many canals 

 conspicuously surrounded by collenchyma. The pores are closed, 

 but gaps in the layer of dermal spicules indicate that they everj- 

 where perforate the dermal membrane. Oscula few, scattered, about 

 3 mm. in diameter ; raised slightly above the surface on short oscular 

 tubes which are scarcely more than rims, 1-3 mm. high. The osculum 

 leads into a canal of corresponding width, which not far from the 

 surface receives several efferent canals. 



There is a very irregular skeletal reticulum of compact spiculo 

 fiber, with abundant free spicules scattered between the fibers. Spic ■ 

 ules of the fibers packed closely and cemented together with a small 

 amount of spongin. Fibers of the reticulum 50-200 \l thick; meshes 

 have about the same range in width. This reticulum extends to the 

 surface of the sponge, and to the lining Avails of the largest canals, 

 in which the abundant tangential spicules form a lining reticulum 

 of spiculo-fiber with rounded meshes. All but the larger canals are, 

 however, surrounded by tissue, collenchymatous in some cases, which 

 lacks the skeletal reticulum and contains only scattered spicules. 

 Since there are numerous canals, many of considerable size, 

 numerous gaps in the skeletal reticulum are produced, occupied 

 either by canals or by canals surrounded by sponge tissue lacking the 

 reticulum. Such interruptions may be thought of simply as larger 

 meshes in the reticulum as a whole; they are of all sizes up to the 

 diameter of the large canals. These many interruptions of different 

 sizes give to the skeleton the appearance of a very heterogeneous 

 structure. 



In the tissue surrounding the canals of considerable size, which, 

 as said above, does not contain any part of the skeletal reticulum but 

 only scattered spicules, it may be seen in thick sections that the 



