191 



formily to the aperture, composed of two difTerent portions, one corresponding 

 with the aureola-like expansion, and one answering to the real aperture. While 

 the former occurs as a uniform chitinous expansion, the latter is provided 

 with a strongly chitinized proximal margin, and with a most chitinized opercular 

 arch, the two arms of which have in their distal half a small rounded process 

 for muscular attachment. The two adoral areas are furnished with two sometimes 

 small and sometimes medium-sized, not very projecting spines of a round or oval 

 section, and they are connected with each other by a very narrow and low, 

 raised margin. The cryptocyst is extremely fuberculated, encircled by a well- 

 developed, crenulated, tuberculous marginal ridge and furnished with closely 

 situated, rather large pores. The two small opesiulai, which are widely separated 

 from the aperture, most frequently circular, more rarely oval, and in which the 

 margin is usually more or less sinuated owing to the very tuberculous condition 

 of the cryptocyst, generally show only a slight difference in size. The opesiular 

 outgrowths both reach the basal wall, which they meet in two closed, ([uadrang- 

 ularly rounded, somewhat distally bent curved lines, which show less difference 

 in size than is usually the case in the species of this genus. The bridge between 

 the two opesiulse is short and broad, very slightly depressed and has con- 

 trary to the rule not infrequently a larger or smaller number of pores in its 

 j)roximal half. 



Spicules occur only in the shape of compasses, of which the smaller are 

 rather strongly l)ent at an angle and also found between the cryptocyst and the 

 covering membrane. Length varying from 0,039 mm. to 0,718™"' 

 Ooecia are not found. 



The avicularia, which are rare and much smaller than the zoa^cia, have a 

 very well-developed proximal cryptocyst, surrounded by a thick, marginal ridge. 

 The mandible is semi-elliptical and has a mandibular cavity in the form of an 

 isosceles triangle. 



Of this species I have examined firstly some colonies, incrusting Tridacna sp. 

 without locality, and secondly^ some one-layered laminae from Torres Straits, be- 

 longing to the Museum of Zoology at Cambridge. 



In some of the polypide-less zoacia I found a narrow, thin-walled, somewhat 

 curved calcareous tube, passing through the whole length of the zou'cium and 

 including the roselle-plales of the two opposite distal walls. It had several lateral 

 branches, which reach the rosette-plates on the lateral walls, and in some cases 

 at least I have found a round or oval opening in the frontal wall of the distal 

 end of the tube. This lube is apparently of the same nature as the one, men- 



