MEMBRANIPORA LACROIXII. 129 



from the margin of the cell, overarching^ as it were, the 

 original membranous covering. 



The present genus may be divided into two principal 

 groups — the first including those species in which the 

 area is wholly roofed in by a chitinous membrane, and 

 the second those in which the covering is converted, to a 

 greater or less extent, into a solid plate. 



It has been proposed to refer to a distinct genus certain 

 forms with a trifoliate or subtrifoliate aperture, taking 

 M. Flemingii as the type. But this character is rather a 

 shadowy one, and the group thus constituted Avould not be 

 sufficiently differentiated from other Memhranipora with a 

 calcareous lamina. 



The terminology employed in describing the members 

 of this genus requires a word of explanation. The area 

 is the portion of the cell surrounded by the raised margin. 

 The aperture is that part of it which is not closed in by 

 a calcareous wall ; and on this is placed the true orifice — 

 a semicircular opening, with a valvular operculum. In 

 the species which have the front wall wholly membranous, 

 the area and the aperture of course coincide. 



The genus Membranipora, as now limited, has a wide 

 range both in space and time, and includes a rich variety 

 of forms. It reaches back as far as the Cretaceous period 

 at least, in which it was represented by a large number of 

 species. It is found in almost all parts of the world. 



a. With a membranous front wall. 



Membranipora Lacroixii, Audouin. 

 Plate XVII. figs. 5-8. 



Flustra Lacroixii, Savigny, figypte, pi. x. fig. 9. 

 Flustra distan.s, HassaU, Ann. N. H. vii. (1841), 369. 

 Flustra Peachii, Couch, 9tli Rep. Cornw. Polytechn. Soc. 81. 



