608 COLLECTIONS FROM THE WESTERN INDIAN OCEAN. 



Islands give the following measurements : acuatc # 2 to *25 by "GOT 

 millim. ; tibiella -2 to "25 by *004 millim. 



J lab. Mozambique ; Marie Louise Island, Amirante group ; tide- 

 marks to 17 fins. 



DESMACIDINIDJE. 



The occurrence of a true Desmacidon in the Indian Ocean is per- 

 haps for the first time indicated by the new species described below. 

 The two species assigned to the genus by Ehlers (' Die Esperschen 

 Spongien') appear to belong respectively to Amphilectus and Olathria. 

 The wide range possessed by species of the new genus Iotrochota is 

 shown also by the occurrence of our two new Australian species, 

 one of them being abundant in both localities. 



RHIZOCHALINA. 



The scarcity of this genus, so common in the tropical waters of 

 Australia, and ■well represented also in the south of that continent, 

 is probably due to the absence of mud from the localities investi- 

 gated ; slightly deeper dredging, clear of the reefs, might be ex- 

 pected to reveal more of this interesting genus, -which had not 

 hitherto been noted from any localities nearer than Ceylon (Carter, 

 Ann. & Mag. N". H. 1880, vi. p. 37, under the name of Desmacidon 

 jeffrcysi). 



29. Rhizochalina pellucida. (Plate LIV. fig. j.) 



Elongate, tapering gradually from base of spouge to summit of 

 fistula. Eistula single, straight. Surface even, glabrous. Consis- 

 tence in spirit soft, brittle ; colour very pale brown ; appearance 

 semitransparent. Body of sponge oval, compressed ; includes foreign 

 bodies. 



Main skeleton a somewhat confused mass of moderately closely 

 felted and irregularly crossing spicules, traversed at intervals by 

 tracts of compact spiculo-fibre, 4-8 spicules broad, running parallel 

 to the surface. Dermal skeleton consisting of long, straight, compact 

 spiculo-fibres, 4-20 spicules broad, branching at acute angles, and 

 thus spreading over the surface ; the intervals occupied by a loose 

 open reticulation of single spicules or of fascicles two or three spicules 

 broad, crossing at various angles. Sarcode pale yellowish brown, 

 subtransparent. Spicule acerate, slightly curved, tapering gradually 

 to sharp points from about middle of spicule ; size '26 by *01 millim. 



Hab. Providence Island, Mascarene group, 19 fms. ; bottom coral. 



A single specimen, 87 millim. (3| inches) long, 12 millim. in 

 greatest basal diameter ; greatest diameter of present end of fistula 

 3 millim. 



In its subtransparency, and in the great thinness of the dermal 

 layer of the skeleton, as well as in its having been apparently sessile 

 by a bulbous base, this differs from all described species of the 

 genus. 



