RETEPORA BEANIANA. 393 



This lovely form, whilst frequently assuming the shape 

 of a cup, exhibits a very considerable variety of habit. 

 The prettiest specimens which I have seen, though small 

 in size, are developed on shells oi Ditrupa from Shetland 

 (Plate LV. fig. 2). They are regular in form, resem- 

 bling a shallow cup, shghtly undulated, and sometimes 

 compressed transversely, and of a glossy ivory whiteness. 

 The little Coralline clasps the slender gracefully curved 

 shell with its spreading base, and rises to a height of 

 about half an inch. In another very characteristic form, 

 from Embleton Bay, there is an intricacy of structure 

 which is altogether wanting in the one which I have just 

 described. The breadth of the specimen is a little less 

 than double the height ; the habit is spreading, the 

 growth irregular. Above the central cup-like depression, 

 which formed the first stage in the growth of the zoarium, 

 the reticulated lamina is enormously developed on one 

 side (or face) only, attaining a height of about an inch, 

 and a width of nearly two inches ; on the opposite side (or 

 face) it is quite rudimentary, so that the cup-like form 

 is lost, and the zoarium has the appearance of a broad 

 foliaceous expansion, variously twisted and curved, with a 

 deeply sinuated margin. In some cases the sinuses are 

 completely inclosed by the union of the opposite edges of 

 the lamina, and a number of funnel-shaped or subcylin- 

 drical cavities are formed, which give a singularly in- 

 volved and labyrinthine character to the whole. In yet 

 another form there is still less trace of a cup, and the 

 zoarium forms a broad, spreading, contorted expansion. 



The minute characters are very constant. The rostrum 

 on the lower margin of the aperture, which supports the 

 avicularium, is never elongated as in R. Couchii, but is 

 always rudimentary. The minute denticles projecting 

 from the inner side of the avicularium are also a distinc- 



