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 would not be 

 sufficiently differentiated from other Membraniporae 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. 



FLUBTRA LACROIXII, Savigny, figypte, pi. x. fig. 9. 

 FLUNK* DISTAKS, Hauall, Ann. N. H. vii. (1841), 309. 

 FLUBTKA PEACIIII. C'oncfi, 9th Rpp. Oornw. Polytechn. Soc. 81. 



