DENDY— HOMOSCLEROPHORA AND ASTROTETRAXONIDA 229 



by Kirkpatrick, Topsent and Lendenfeld in the genus Plakinastrella possesses trisenes, 

 but, on the contrary, all of them belong to the Homosclerophora. 



The genus to which Dercitopsis is most closely allied is not, in my opinion, Plakina- 

 strella, but Plakortis [Schulze 1880]. Indeed it is perhaps not easy to separate the two 

 satisfactorily, but Schulze, in his original diagnosis of the genus Plakortis, emphasised the 

 fact that tetract spicules were wanting, only triacts and diacts being developed. We may 

 take this as the basis of the generic distinction. Plakortis also seems to be more primitive 

 than Dercitopsis as regards canal system, but in this respect the gap seems to be bridged 

 over to some extent by Dercitopsis clathrata (Kirkpatrick). 



The presence of radially arranged small oxea at the surface must be abandoned 

 as part of the generic diagnosis of Dercitopsis, for, though such a layer is present in 

 D. ceylonica Dendy, D. clathrata (Kirkpatrick) and D. minor n. sp., it is absent in 

 D. oxeata (Topsent) and (apparently) in D. mammillaris (Lendenfeld). The last named 

 species is remarkable in another respect, for the spiculation includes only oxea and 

 calthrops, triods being completely absent. 



1. Dercitopsis minor n. sp. 

 (Plate 44, fig. 1 ; Plate 45, fig. 1.) 



The sponge (Plate 44, fig. 1) forms irregular, rounded, cushion-like masses, encrusting 

 pieces of rock, &c. The margins are broadly rounded and may project considerably beyond 

 the base of attachment, and become tucked in, thus tending to envelope the support. The 

 maximum dimensions of the largest specimen (R.N. XLii. 6) are as follows: — Length 78 mm., 

 breadth 57 mm., thickness about 16 mm. The surface is smooth but rather uneven; 

 subgiabrous and minutely punctate. The colour of the surface (in spirit) varies from 

 light brown to dark slate grey ; internally it is pale yellowish. Vents of moderate size, 

 up to about 3 mm. in diameter, each with a prominent, membranous collar ; few in number 

 and scattered singly on prominent portions of the upper surface. Inhalant pores closely 

 scattered all over the surface. Texture firm and compact. 



The skeleton is a dense feltwork of loose spicules, quite irregularly arranged except 

 at the surface, where very small oxea are placed more or less at right angles to the suiface 

 to form a dermal layer. 



Spicules : — (l) Calthrops (Plate 45, fig. 1 a), with smooth, sharp-pointed rays measuring 

 about 0'037 by 0*005 mm. in a well grown specimen, but varying a good deal. 



(2) Triods (Plate 45, fig. 1 h), dijEfering Jfrom the calthrops in the absence of one ray. 

 Perhaps, on an average, the rays of the triods are more slender than those of the calthrops, 

 but slender-rayed forms of both occur. 



(3) Oxea (Plate 45, fig. 1 c), fusiform, slender, slightly curved, gradually sharp- 

 pointed, almost always with a kink or enlargement in the middle. Size variable, averaging 

 in the deeper parts of the sponge, say, about 0"1 by 0"004 mm., though often much more 

 slender. The largest seen, and that only once, measured only about 0'19 mm. in length. 

 The small dermal oxea measure up to about 0"04 mm. long and are of proportionate 

 thickness. Intermediate sizes between these and the deeper oxea are abundant. The 

 oxea are far more numerous than both triods and calthrops together. 



