132 MEMBRANIPORID^E. 



Zooecia subovatc, produced and attenuated below; area 

 oval, more or less elongate, occupying about two thirds 

 of the front of the cell, with a membranous covering ; 

 margins thin and plain, bearing a single acuminate 

 spine below the aperture, bending inwards, and occa- 

 sionally a tall and very slender erect spine on each 

 side near the top, or armed with numerous delicate 

 spines (about 18), the central one below being larger 

 and stouter than the rest, and the two uppermost 

 standing erect. Zoarium usually assuming a dendritic 

 growth, giving off branches consisting of lines of cells 

 two or three abreast, or in single series, which anasto- 

 mose freely. 



Var. a (fossaria) . Zooecia elongate-oval ; area occupying 

 the whole of the front of the cell ; a single stout conical 

 spine at the bottom. Zoarium spreading over the stems 

 of plants in brackish water, and forming friable, irregu- 

 larly shaped, sponge-like masses. 



RANGE OF VARIATION. M. monostachys affords a striking 

 illustration of the remarkable diversities in habit and 

 general appearance that may rank under one and the 

 same specific type. In its early condition it forms a con- 

 tinuous expansion, the cells being disposed in somewhat 

 radiate fashion. But the zoarium does not long maintain 

 this simple character ; it is soon resolved, as it were, into 

 lines of cells, in which two or three are ranged abreast ; 

 and these again give off single series, from which in time 

 other compound series may originate ; and so the rami- 

 fication proceeds, until a large and complex and very 

 beautiful dendritic growth is the result. 



The differences in the number of the spines with which 

 the margin of the cell is armed are also great, and they 

 affect very materially the appearance of the species. The 

 acuminate spine at the bottom of the area is tolerably 

 constant ; it is always present on some of the cells in a 



