MUCRONBLLA VAEIOLOSA. 367 



elongated, lozenge-shaped, or nearly oval, and much nar- 

 rowed below. The mucro is sometimes bidentate, and 

 occasionally almost abortive. The greatest amount of 

 variation occurs in the marginal areolation, which in some 

 cases almost disappears, whilst in others it exists in such 

 an exaggerated condition as completely to change the ap- 

 pearance of the species. Specimens in which this occurs 

 are often of very great beauty, the areolation occupying a 

 large portion of the surface of the cell, and a small cen- 

 tral space only being left smooth. In other cases the areo- 

 lation is very partial, in others extremely obscure ; in 

 others it is represented by mere superficial ribs. A variety 

 occurs in which the cells are very flat, the peristome is not 

 elevated, and the anterior margin scarcely prominent. In 

 another and very pretty form the cells are not much pro- 

 duced, are bordered by a white sinuous line, and speckled 

 with very distinct, minute white granules on a grey ground. 

 With age a thin white crust often forms over the primitive 

 cell-wall, which is of a delicate greyish colour. Very old 

 specimens present a flat surface, and the walls are thick- 

 ened and strongly granulated. 



HABITAT. On stones and shells from deep water. 



LOCALITIES. Common, and generally distributed. It 

 has not occurred in Shetland, nor on the Scandinavian 

 coast. The Minch, Hebrides (Norman) : St. Andrews, 

 deep water (Dr. M'Intosh) : Sana Island, 40 fathoms 

 (Hyndman) : Birterbuy Bay (G. S. B.) : off the Deadman, 

 60 fathoms, on Pinna (T. H.) : &c. 



GEOGRAPHICAL DISTRIBUTION. Roscoff (Joliet) : New 

 Zealand (F. W. Hutton) : Adriatic, rare (Heller) : mouth 

 of the Jenesei (subfossil) (F. Schmidt). 



RANGE IN TIME. Coralline Crag, on shell (S. Wood) : 

 Middle Pliocene (Coralline Crag and Red Crag, part) (A. 

 Bell): Austro-Hungar. Miocene (Reuss). 



