212 



MICROPOEELLIDiE. 



Var. a {tJujreoplwra). Upper half of the zooecia in front 

 occupied by a scutiform area, in the centre of which is 



Fiff. 10. 



Fig. 11. 



Primary cell. 



Centre of colony. 



the lunate pore, and on either side a single row of punc- 

 tures, which also extends across the front of the cell im- 

 mediately below the mouth. {Busk.) (PI. XLV. fig. 1.) 

 Var. jS (vitrea). Zooecia glassy and transparent, and en- 

 tirely destitute of the stellate pores. In this variety 

 the cells are unusually small, and the two anterior 

 spines are forked. 



Range of Variation. This is a much less variable spe- 

 cies than the last. The stellate pores are occasionally 

 wanting altogether, and the walls of the cell are smooth 

 and glassy. In some cases they form a single line along 

 the sides, and a small group above the central pore ; in 

 others, again, they are more diffused, and the smooth, 

 central space is proportionately reduced. In old specimens 

 the walls are thickened, the surface is reticulated, and the 

 median pore surrounded by an elevated border. 



Occasionally the two anterior spines are forked, a 

 character which is constant in some species. According 

 to Busk, in all South-American specimens the cells are 

 much larger than in the European form — a difference which 

 is no doubt due to climatal influences. 



