272 MYRIOZOID.^. 



Colonies forming white circular patches, often with a 

 silvery lustre. 



Var. a [cornuta) . With a stout tubular process on each 



side of the mouth. 

 Var. /3 [incrassata) . Walls thickened and opaque. 

 Var. 7 [tuber culatd). With a number of tubercles on 



the front of the cell, and often a strongly developed 



umbo below the orifice^. 



Range of Variation. In what may be regarded as 

 its normal condition, the cells of Si. hyalina are glassy 

 and transparent, of thin substance, and most delicate 

 texture. 



A variety, however, is of common occurrence in which 

 the walls are much thicker, opaque, and of a dirty white 

 colour. The cells, except those which carry the ovicells, 

 are usually subcylindrical, elongated, and very slightly 

 bulging at the sides, but occasionally of a shorter and 

 broader make. In some cases the cell-wall slopes rather 

 steeply up on each side towards a central keel, which 

 passes from the orifice, where it rises into a prominent 

 point, to the base of the cell. A ridge-like umbo is 

 also frequently developed immediately below the inferior 

 margin. The orifice presents itself in two very diff'erent 

 conditions : in one it is ample and simply orbicular ; in 

 the other there is a marked sinus on the lower margin, 

 which is sometimes broad and open and sometimes very 



* Busk, in a paper on new Species of Poljzoa from Kerguelen's Island 

 (Ann. N. H. for Feb. 1876, p. 116), describes a yariety conferta, distin- 

 guished by " tke crowded and confused growth of cells and ovicells in the 

 central portion of the patch," giving it the appearance of a Cellepora, and by 

 the wide, patulous mouth. But the first of these characters has certainly 

 no claim to be regarded as varietal. It belongs to the ordinary form of the 

 species, and is one of its most striking features. It is associated with the 

 wide, patulous, orbicular mouth, and also with the smaller sinuated aper- 

 ture. 



