268 MYRTOZOID.E. 



membrane, and appear as glittering spots of a pale golden 

 colour. With age the walls thicken and the foramina are 

 sometimes obliterated, the surface being uniformly gra- 

 nular. In a variety from Shetland the front wall of the 

 cells is almost smooth and glossy, and covered by a network 

 formed by a delicate beaded line (Plate XLII. fig, 4), 

 When old and worn the red tint is lost^ the walls are much 

 thickened, and of a dirty whitish colour, and coarsely punc- 

 tured. Postpliocene examj)les from Canada present exactly 

 this appearance, and hardly differ, except in the larger size 

 of the cells, from old specimens dredged in Scotland. 



Smitt has remarked that a small avicularium is some- 

 times present on the lower margin, between the primary 

 and secondary apertures. On British specimens I have 

 never seen any trace of such a structure ; but on a form 

 from the Gulf of St. Lawrence, which in all other respects 

 (except, perhaps, the size of the cells) agrees with S. 

 sinuosa, an avicularium with a rounded mandible occurs, 

 occupying much the same position as in Porella concinna. 

 I have described this form as var, a. (armata). 



The central pore on the front of the ovicell frequently 

 shares the fate of the foramina in the cell-wall, and is 

 obliterated by the calcareous overgrowth. 



Habitat. On shells and stones from shallow to deep 

 water (150 fathoms) . 



Localities. Shetland (Barlee) : ibid., on stone and 

 shell. Outer Haaf (A. M. N.) : west coast of Scotland 

 (T.H.). 



Geographical Distribution, Spitzbergen, 30-50 fa- 

 thoms ; Finmark, frequent (Smitt) : Greenland (North- 

 Germ, Pol, Exped,): lat. 72° 55' N,, long, 37° 57' 68" 

 E., 150 fathoms (Dutch Arctic Exped, 1878): Gulf of 

 St. Lawrence, normal and var. a (Dawson). 



Range in Time. Canadian Postpliocene deposits 

 (Dawson) . 



