166 CEYLON PEARL OYSTER REPORT. 



Monaxonida, Mr. Ridley and I suggested that Rhizockalina and Oceanapia should 

 lit' united in one genus, and in a subsequent paper (10) I have carried out this 

 suggestion. Our reason for this proposal was that at one of the " Challenger " stations 

 (188, off New Guinea) specimens with and without sigmata appear to occur together, 

 and are so closely similar in external appearance that they cannot he distinguished 

 otherwise than microscopically. It seems equally reasonable, however, to suppose 

 that the two genera actually occur together in this locality, or that there has been 

 some confusion in the sorting out of the specimens. We can hardly suppose that the 

 same species sometimes occurs with, and sometimes without, sigmata. I therefore 

 propose to return to the arrangement originally adopted in the " Challenger" Report, 

 substituting, for the reason above given, the name " Phlceodictyon" for " Rhizochalina." 



Neither Mr. Carter nor I have been able to find sigmata in the Ceylon specimens, 

 though I have myself examined about half-a-dozen microscopically. 



One of Professor Herdman's specimens has the body, which varies much in shape 

 and is often very irregular, up to 48 milliins. in maximum diameter. As pointed out 

 by Mr. Carter, the fistular processes are long and may be ramified. They sometimes 

 appear to be naturally closed at the extremity (except, perhaps, for small pores), and 

 sometimes open. In nearly all cases they have been broken off short. 



R.N. 81, 90, 237 (all from deep water off Galle and onwards up West Coast ot 

 Ceylon); 260; 363 (fistula? only, Stat. I., hauls 1-4, January 31, 1902, Colombo to 

 Negombo, 12 to 20 fathoms). Also other unnumbered specimens and loose fistula'. 



Histoderma, Carter. 



Phlceodictyimt' with usually diactinal megascleres and microscleres in the form of 

 isochelas, to which others may be added. 



Au examination of Mr. Carter's own preparations of his Phlceodictyon singa- 

 porense (54) lias revealed the presence of small palmate isochelse. I therefore propose 

 to place this species, together with a closely related Ceylonese form, in the genus 

 Histoderma. 



The presence of isochelas in this genus is of great interest and necessitates the 

 inclusion of the Phlceodictyime amongst the Desmacidonida?, the two species referred 

 to forming an important connecting link between Phlceodictyon and Oceanapia on 

 the one hand and Desmacidon on the other. 



Histoderma vesiculatum, n. sp. Plate XL, figs. 8, 9. 



This remarkable sponge is represented in the collection by two fragments of about 

 equal size and closely similar appearance. Each consists of an elongated cylindrical 

 body inflated at irregular intervals to form oval vesicles. In one specimen there are 

 two distinct vesicles connected by a short cylindrical piece, and from the larger of the 

 two, which is broken across at the end, another short cylindrical piece is given oft' as 



