MEMBRANIPORELLA NITIDA. 201 



spaces^ ou 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 avicularhim above it on each 

 side, pointing upwards and outwards. 



Primary zocec'nmi 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. 

 %. 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 in 



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 regidar 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 speaks of very beautiful specimens which be had 

 obtained from Budleigb Saherton, "the whole fabric of which had a metal- 

 lic appearance ; the tiny ribs seemed made of steel. " — Pop. Hht. p. 319. 



