SILICIOUS AND HORNY SPONGES WILSON. 457 



regular reticulum. In each mesh of the reticulum there is some- 

 times only one, more often a number (two to four), of the discotri- 

 aene cladomes. 



In correlation with the increase in the number of pores, the sub- 

 dermal cavities, which in the specimen assigned to the type had al- 

 ready begun to fuse in rows, here constitute tubular channels, about 

 250 \i wide, which extend parallel to and just beneath the surface 

 membrane. Such channels have obviously been produced by the con- 

 tinued formation of small subdermal cavities and their fusion in 

 rows. (They are best seen in thick tangential sections of the surface 

 region, examined from the inner side.) 



Again, the main afferent and efferent canals have acquired a cer- 

 tain regularity of arrangement in correlation with the lamellate shape 

 and the differentiation of the two faces as oscular and poriferous. 

 They are of about the same size and they both pass radially into the 

 sponge interior from the respective faces. 



As to the skeleton of these lamellate specimens, the long slender 

 rhabds are especially abundant in the marginal region. Otherwise 

 the only skeletal difference from the type concerns the microscleres. 

 These are here less uniform, ranging commonly from 8 to 25 pi in 

 length. 2 to 3 pi thick at the middle ; not infrequently reaching a size 

 of 40 by 4 pi. All the spicules are minutely roughened, thicker in 

 the middle and tapering toward the ends; ends strongylate in the 

 smaller spicules, oxeate in the larger but never sharp; in the larger 

 spicules, the two halves commonly bent at a slight angle; the larger 

 spicules sometimes distinctly centrotylote. While the presence in 

 considerable numbers of the very small rhabds, often narrowly 

 ellipsoidal in shape, catches the eye, all sizes and shapes of these 

 spicules are intermingled and intergrade so that they can not be 

 separated; the distribution is the same as in the specimen assigned 

 to the type. Thus as in the case recorded by Topsent, Discodermia 

 rami f era (Topsent 1904, p. 58), the microscleres here form a single 

 category which includes both the microrhabds and the microxeas of 

 the conventional generic diagnosis as given for instance by Sollas 

 (1888) and Lendenfeld (1903,1906). 



Holotype.— Cat. No. 21260, U.S.N.M. 



Genus JEREOPSIS O. Schmidt (1879). 



Jereop.fis O. Schmidt, 1S79, p. 20. — Lendenfeld, 1903, p. 133. 

 Xeosiphonia Sollas, 1888, pp. 290, 334. 



The desma is tetracrepid; crepis, in some species at least, an 

 amphitriaene. The ectosomal triaenes are dichotriaenes or tricho- 

 triaenes. "With streptasters that vary from the spiraster to the 

 amphiaster type. 



81709—25 13 



