A REVISION OF THE GENUS STEGANOPORELLA. 265 



form, having four well-developed teeth and a series of 

 secondary sclerites. The young operculum shows the main 

 sclerite only. The smaller sclerites consist of a basal 

 sclerite, an irregularly arch-shaped sclerite which is roughly 

 concentric with the main sclerite, and additional sclerites 

 starting from these two. One of these may start from the 

 arch-shaped sclerite opposite each of the four teeth, and 

 unite with the main sclerite in such a way as to pass beneath 

 its projecting edge and to form a buttress-like support for 

 the tooth. Although the pattern of the opercula is very 

 variable even in the same colony, there is usuall}^ a recognis- 

 able difference between these opercula and those of the 

 Vincularian forju, in which there is commonly a more or less 

 transverse secondary sclerite and a s^'stem of subparallel 

 longitudinal sclerites between this and the basal sclerite. 

 The proximal projections of the main sclerite of var. 

 magnifica are usually hook-shaped on their outer side, but 

 they commonly have a lobe on their inner side as well. The 

 most striking difference between this and the Vincularian 

 form is, however, the existence of numerous fine denticula- 

 tions on the outer side of the base of the main sclerite. 

 These appear to be constant in this variety, but I have seen 

 no trace of them in the typical S. neozelanica. 



The size of the opercula is rather uniform. The average 

 diameter is about 550 in. 



I am very doubtful whether this form should be regarded 

 as a distinct species, in which case Busk's MS. name 

 magnifica, affixed to the specimens fron Tongatabu, might 

 be regarded as the specific name. The difference in habit 

 between the Vincularian and the encrusting forms seems to 

 be constant. Some of the other differences, such as the 

 form of the basal wall, the boundaries of the tube, and the 

 shape of the lateral recesses, are directly connected with the 

 habit. Others, and in particular the denticulation of the 

 opercula, the pattern of the secondai-y sclerites, and the shape 

 of the median process, are not obviously connected with the 

 habit. 



