PORELLA MINUTA. 327 



margin ; primary orifice semicircular ; secondary orifice 

 arched above, slightly contracted below, the sides 

 straight, somewhat inclined inwards, on the lower lip 

 a rounded prominence (often with a crenated rim) 

 supporting an avicularium, with semicircular mandible ; 

 the cell-wall sometimes much raised and thickened 

 round the orifice, sometimes rising into a mucro in 

 front. Ooecia subimmersed, rounded, smooth, or 

 granular. 

 Colonies forming ''very small roundish patches." 



Range of Variatiox. In the few specimens I have had 

 the opportunity of examining the characters are on the 

 whole very constant. The littoral form, however, of 

 which I have received specimens from Mr. Peach, difi'ers 

 in some points from the deep-water form. It has the 

 cell and ovicell smooth and of a delicate whiteness, 

 while in the latter they are granular and comparatively 

 coarse; the peristome is not thickened; the circular 

 prominence which supports the avicularium is not so 

 apparent, being partially concealed by the overhanging 

 prseoral mucro, which is strongly developed, and it has a 

 plain rim, whereas in the deep-water variety the border is 

 very prettily crenated. In the littoral specimens also the 

 arched top of the orifice is carried across the front of the 

 ovicell (Plate XXXVI. fig. 6). 



In some cases the mucro is well developed ; in others 

 it is absent, and there is merely a swelling below the 

 mouth. 



The primary orifice, as seen in the marginal cells, is 

 semicircular, without any elevation of the peristome. 



Habitat. Between tide-marks, on stones, shells, &c., 

 and from deep water. 



Localities. Shetland, very rare; Guernsey (A. M. N.) : 

 Wick, between tide-marks (C. W. P.) : Antrim (Hvnd- 



