MICROPORELLA CILIATA. 207 



Zocecia ovate, obscurely six-sided, convex, separated by 

 well-marked sutures, silvery, and more or less dis- 

 tinctly punctate, or white and granular, quincuncial, or 

 disposed in radiating lines ; orifice arched above, with 

 a slight rim, the lower margin straight, the upper with 

 from five to seven long spines; median pore lunate, a 

 little below the orifice, frequently borne on a promi- 

 nent mucro, or sometimes almost hidden behind it ; on 

 one side or the other a large avicularium, with acute 

 mandible, directed obliquely upwards, which is often 

 prolonged into a slender vibraculoid spine. Ooecia 

 globose, occasionally areolated round the base, minutely 

 punctate or granular ; two spines visible in front of it. 



Var. personata. Ooecium extended in front, so as to form 

 a wall-like inclosure round the orifice (hooded) . 



The colonies originate in a small oval cell, with an aper- 

 ture occupying the entire front, which is covered in 

 by a membrane, and surrounded by numerous slender 

 spines. 



RANGE op VARIATION. This species shows a tendency to 

 variability in almost all portions of its structure, except 

 the orifice of the cell, but may always be easily recognized 

 by the semicircular mouth, the median pore, and the 

 lateral avicularium. The texture and sculpture vary with 

 habitat and age. The delicate form, with sheeny frosted 

 surface, occurs principally on weed, between tide-marks, 

 or in shallow water. In deep-water specimens the cells 

 are usually granulous, of denser substance, and a dull 

 white colour. When old, they are less convex, much 

 thickened, and coarse in appearance. 



Shape of cells : this is commonly pretty regularly ovate ; 

 but sometimes the figure is broad and six-sided. Arrange- 

 ment : usually in quincunx, rarely in radiating lines. Size : 

 extremely variable: specimens occur (PI. XXVIII. fig. 4) in 

 which they are fully a third smaller than in the ordinary 



