168 MEMBRANIPORID^. 



Range of Variation. There are very considerable dif- 

 ferences in the form and proportions of the cells, cor- 

 responding generally with those which I have already 

 described in M. Rosselii. The lozenge-shape predominates ; 

 but the elongate form, wide and arched above, and nar- 

 rowing off from an angle on each side about the middle 

 of the cell, is far from uncommon. 



The variety a differs from the typical form as described 

 by Norman chiefly in the character of the ovicell. The 

 aviculariura, too, is situated on the lamina instead of be- 

 tween the cells ; and the margin is more strongly developed 

 and coarsely beaded. 



Habitat. Incrusting shells and stones in shallow and 

 deep water. 



Localities. Shetland, common, 40-170 fathoms (A. 

 M. N.) : Wick, var. a (C. ^^' . P.). 



Geographical Distribution. South Labrador (Pack- 

 ard) : St. Lawrence (Dawson) : Bahusia, common, 10-100 

 fathoms (Smitt) : Sj)itzbergen, Rode-Bay, 40 fathoms 

 (Goes) : Greenland, Holsteinborg Harbour, 7-35 fathoms 

 (' Valorous' dredgings) : Bergen (A. M. N.). 



Range in Time. Coralline Crag, very abundant on 

 Terebratula grancUs and other shells (S. W.) : Red Crag 

 (A. Bell). 



The present species is closely allied to M. Rosselii. In 

 the absence of the ovicells and of the avicularia, which 

 are very sparingly developed, there may be some difficulty 

 in distinguishing the two. In the latter the surface of 

 the zoarium is flatter, and neater in appearance than in 

 M. trifolium, the zooecia are smaller and narrower, the 

 aperture generally occupies a smaller proportion of the 

 area, the margin is more elevated round the top of the cell, 

 the lamina is thicker and more depressed. In M. tri- 



