430 MOLLUSCA. 



composed of only one layer of cells, the mouths of which 

 open on the convex surface. Lastly, we have the family of 

 the Vinculariadce, typified by Vincularia itself (fig. 269, i), 

 in which the polyzoary is erect, rigid, and branched, the 

 cells disposed alternately round an imaginary axis, and 

 having a raised border in front. The undoubted species of 

 the genus are Cretaceous, Tertiary, and Eecent ; but it seems 

 probable that some of the Palaeozoic Polyzoa may ultimately 

 be referred to this family, if not to this genus. 



Passing on next to the series of the Cyclostomatous Polyzoa, 

 with their tubular cells and terminal cell-mouths, we need 

 merely mention the family of the Crisiadce, represented in 

 Tertiary deposits by the genus Crisia (fig. 269, F). A much 

 more important family is that of the Idmoneidce, in which 

 the polyzoary is erect, simple, or branched, the branches 

 usually round, and sometimes anastomosing with one an- 

 other. In the genus Homer a (fig. 269, K) the cells open 

 on one side only of the branches, the mouths being placed 

 in somewhat rhomboidal spaces marked out by wavy anas- 

 tomosing ridges. The species of this type are Cretaceous, 

 Tertiary, and Eecent, but some Palaeozoic forms may possibly 

 find a place here. In Idmonea itself (fig. 269, J), on the 



Fig. 271. Pustulopora (Entalophora) cellarioides, natural size and enlarged. Jurassic. 



other hand, the cells are disposed in transverse or oblique 

 rows on each side of the front faces of the branches, which 

 are divided mesially by an angulation or longitudinal keel. 

 The genus ranges from the Cretaceous to the Tertiary. In 

 the third genus, Pustulopora (Entalophora), the tubular cells 



