SCHIZOPORELLA BIAPERTA. 257 



The ovicells of Hippo thoa diver gens are not described 

 by Smitt; but on Algerian specimens in my possession, 

 which exhibit the characters of this variety, they are 

 developed abundantly, and they are identical with those 

 figured by Smitt for the biaperta form. On the whole, I 

 can see no reason for making two species, and I therefore 

 unite the varieties under Michelin's original name. 



Besides the differences in the contour of the mouth, to 

 which I have just referred, there are others in the super- 

 ficial characters. 



In British specimens the walls are solid and compact, 

 perfectly smooth, of a glossy white colour, and imperforate. 

 In the Algerian examples the walls are thinner, and the 

 texture much more delicate. The Crag form, figured by 

 Busk, has a perfectly smooth surface, and has an avicula- 

 rium on one side of the mouth only. In recent specimens 

 there are more usually two, one on each side ; but in some 

 cases a single oral avicularium occurs. 



Smitt assigns to S. biaperta a row of punctures round 

 the margin, which I have not observed in British speci- 

 mens. Their absence may be due to a different degree of 

 calcification. 



The number of the large, mamillated avicularia varies 

 in a remarkable manner. Sometimes they are scarcely to 

 be met with ; sometimes they occur in wonderful profu- 

 sion, as on the Algerian specimens, in which almost every 

 cell, in certain portions of the colony, bears a large mound, 

 covering the greater part of the front surface. The avicu- 

 laria are placed on the side of these mound-like promi- 

 nences, the extremity of the " beak " just reaching to the 

 summit. The mandible is broad at the base, and tapers 

 off rapidly to a point. 



In this and the allied species, S. anna/a, we have a 

 striking illustration of the variability of the avicularium ; 



