362 



REVISION OF THE MONAXONID SPONGES, 11., 



description, affords practically indisputable reason for believ- 

 it to exemplify the species to which the name R. petrosia was 

 intended to apply. The question, as to which of these speci- 

 mens is to be considered the type of the species, appears to me 

 one that might be decided quite well by the toss of a coin ; but 

 as the latter best accords with the name, I propose that it be 

 taken as the type — the species thus requiring to be called 

 Phhieodictyon petrosia. 



An adequate description of Ph. petrosia cannot, at present, 

 be given, as the small fragment at my disposal consists of 

 scarcely more than a portion of the rind. As far as can be 

 judged from this, however, the species is distinct from any 

 other that has been described. The rind is 

 usually well-developed, attaining to a thick- 

 ness of nearly 2 /x ; its skeleton consists of 

 an approximately rectangular, coarse reticu- 

 lation of stout fibres, measuring up to 150 /x 

 in diameter, composed of closely and not 

 very regularly packed oxea uncemented by 

 spongin. The skeleton bears a marked re- 

 semblance to that which is characteristic of 

 many species of Petrosia. A dermal skele- 

 ton proper, external to the rind, appears to 

 be absent. What little of the main skeleton 

 is shown, consists mainly of scattered spicules 

 exhibiting a tendency towards an arrange- 

 ment in an irregular subrenieroid manner; 

 but there also occur, at intervals, very stout 

 fibres, similar to those of the rind, which 

 apparently belong to inwardly-directed ex- 

 tensions of the latter, such as have been noticed in Ph. ramsayi. 

 The spicules, which are the same in the rind as in the main 

 skeleton, are somewhat angulately curved and abruptly sharp- 

 pointed oxea, ranging in length from 1 30 to 165/x, and measuring 

 seldom less than 6 and not more than 8*5 /x in stoutness. 

 Loc. — Port Jackson. 



Fig. 9. 



Phlceodiclyon petrosia 

 Oxea. 



