SMITTIA MARMOREA. 351 



rather large punctures round the border ; surface 

 coarsely granular ; orifice suborbicular, with a loop- 

 like sinus on the lower margin ; peristome raised, not 

 thickened^ a very broad plate or denticle set deeply 

 within the mouth ; a little below the orifice an elongate 

 avicularium, usually immersed, with a pointed mandible 

 directed downwards. Ooecia globose, punctured, im- 

 mersed in the older cells, with a narrow raised border 

 round the base. 



Habitat. On stone and shells. 



Localities. Cornwall, probably (T. H.) : Guernsey 

 (A. M. N.) 



The walls of the cells are thick ; and the surface, which 

 is covered with large granules, has a somewhat polished 

 and marble-like appearance. The cells are moderately 

 convex, elevated towards the mouth, and regularly punc- 

 tured round the margin. The peristome is raised, espe- 

 cially in front, where it is hollowed into a rather large 

 loop-like sinus. The avicularium is immersed and in- 

 conspicuous ; it is occasionally absent. The young cells 

 are perfectly smooth, and with the surface entire. 



S. marmorea and S. cheilostoma are kindred forms, having 

 many features in common, but still exhibiting differences 

 of such a kind as entitle them to be accounted distinct. 

 In the former the cell-wall seems never to be punctured, 

 except round the margin, where there is a very conspi- 

 cuous line of perforations : the surface is coarsely gra- 

 nular ; in S. cheilostoma it is uniformly covered with 

 pores over its entire extent, and there is no special row of 

 them round the edge. The cell of S. marmorea in its 

 earliest stages is destitute of punctures ; so that the dif- 

 ference between the two forms in this respect is not due 

 to mere age and calcification. In the latter the wall of 



