MEMBRANIPORELLA NITIDA. 201 



spaces, on each side (three to eleven or twelve, or even 

 more) ; mouth semicircular, frequently a mucronate 

 process, immediately below it, at the top of the me- 

 dian line ; oral spines 4-6 : a raised avicularium at the 

 base of many of the cells, or sometimes two ; man- 

 dible acute, directed obliquely downwards. Ocecia 

 subglobose, smooth or minutely granular, often of a 

 pearly-white colour; an avicularium above it on each 

 side, pointing upwards and outwards. 



Primary zocecium broad, ovate ; area closed in by a mem- 

 brane ; four tall erect spines at the top, and ten round 

 the margin, slender, and bent inwards (Plate XXVII. 

 tig. 6). 



Polypides very delicate, bluish-white, with 14-16 tentacles. 



Colonies forming subcircular, shining patches. 



RANGE OF VARIATION. The proportions of the cell and the 

 number of the rib-like processes composing the front wall 

 are liable to much variation. An elongated oval form from 

 Devonshire has sometimes as many as twelve or thirteen 

 of the latter on each side, placed closely together. This is 

 the maximum ; and in the same colony the number ranges 

 from seven to thirteen on each side. Varieties, however, 

 occur in which it is reduced to four or even three ; and iii 

 these cases the ribs are much broader and more widely 

 separated than in the ordinary form. 



In the Devonshire specimens* just referred to the cell 

 is unusually long, and very regular and graceful in shape ; 

 the surface is also remarkably bright and silvery. In some 

 cases a broader and flatter type is met with ; and in others, 

 again, the cell is contracted above and expanded and well 

 rounded off below. There are also differences in the 

 degree in which the ribs are united and consolidated, and 

 in the breadth of the central band or sternum. 



* Dr. Landsborough opeaks of very beautiful specimens which he had 

 obtained from Budleigh Salterton, " the whole fabric of which had a metal- 

 lic appearance ; the tiny rib* termed made of steel." Pop. Hist. p. 311*. 



