FLUSTRA. 345 



also placed a little more posteriorly. An important differ- 

 ence, however, between these very similar species consists 

 in the reniform pedunculate operculum, which projects in 

 front of the mouth of the cell. Although this organ exists 

 in a great variety of form in many species of Cellulariadce, 

 and is particularly well developed in the common C. reptans, 

 I am not aware that it has hitherto received the attention 

 it would seem to deserve in the distinction of species. This 

 process does not arise from the edge of the cell, but from 

 the wall of the cell a little beyond the margin, and it usually 

 appears to be tubular at its origin. It assumes various 

 forms, some very fantastic, and increases in size as the cell 

 becomes older, so that in the older cells at the bottom of 

 the branches it almost entirely covers the mouth." 



Genus XVI. FLUSTBA, Sea-Mats, Linnaeus. 



Gen. Char. Polypidom plant-like, membranous, frondose or 

 crustaceous, formed of cells arranged quincuncially in several 

 series, and in one or two layers : cells in juxtaposition, more or 

 less quadrangular, flat, with distinct border : the aperture trans- 

 verse, semi-lunar, valvular, subterminal. Dr. Johnston. 



