CHEILOSTOMATA. 635 



drical, and the zocecia disposed round an imaginary axis. The type- 

 genus is Cellaria (=Sa/icomaria), in which the surface (fig. 457, c) 

 is divided into rhomboidal or hexagonal spaces, representing the 

 front walls of the cells ; and irregularly disposed avicularia are present. 

 The species of this genus range from the Chalk to the present day. 



The family of the Vinculariidce is typified by the genus Vincularia, 

 the species of which are Cretaceous, Tertiary, and Recent. In this 

 genus (fig. 457, 1) the polyzoary is erect, branched, and rigid, the 

 zocecia being disposed alternately round an imaginary axis, and hav- 

 ing a raised border in front. 



The great family of the Membraniporidce includes forms in which 

 there is a calcareous or corneo-calcareous polyzoary, composed of 

 horizontal and contiguous cells, the colony forming an incrusting 

 expansion, or in some cases giving rise to an erect growth. The 

 zocecia are generally separated by raised margins, the front wall 

 remaining more or less uncalcified. Owing to the membranaceous 

 structure of the anterior walls of the cells, the front of the zocecia in 

 fossil specimens always appears to be more or less largely open. In 

 the extensive genus Membranipora (fig. 481), the cells are surrounded 

 by a well-marked elevated border, 

 the space included within which 

 is technically spoken of as the 

 " area." The entire " area " may 

 be occupied by a horny mem- 

 brane, in which the true cell-aper- 

 ture is pierced ; and when this is 

 the case, the entire "area" in the 

 fossil condition is open. In other 

 cases a larger or smaller part of the 

 membrane occupying the " area " . F[ s- ^.—Membranipora oceani, show- 



!•/-•,, . in S tne sub -terminal mouths of the cells. 



may be calcified, the remainder Upper Cretaceous. 

 being left permanently soft. In 



these cases the " area " of the cell in fossil examples exhibits a more 

 or less extensive deficiency or " aperture," which is always of greater 

 size than the proper cell-mouth itself. The polyzoary in the genus 

 Membranipora is always incrusting, and the numerous species of the 

 genus range from the Chalk to the present day. A similar geological 

 range is possessed by the genus Biflustra, in which the polyzoary is 

 generally erect and foliaceous, and is typically composed of two layers 

 of cells placed back to back. 



In the family of the Microporidce the polyzoary is incrusting, or 

 sometimes free and unilaminar, and the zocecia resemble those of 

 the preceding family in being surrounded by elevated margins (fig. 

 482, a), but the front wall of the cells is completely calcareous. 

 The genus Micropora (fig. 482, a) comprises forms in which the 



