638 MOLLUSCOIDEA. 



which the polyzoary is erect and calcareous, and usually has the 

 form of a reticulate or fenestrated expansion, which is adherent by 

 means of an incrusting base. The zocecia are developed on the 

 front face only of the zoarium, the dorsal surface being smooth or 

 striated, and carrying avicularia supported upon tubercles. The 

 cells are closely united or immersed, and on the lower margin of the 

 cell-mouth is a prominent rostrum carrying an avicularium. The 

 species of Retepora range from the Chalk to the present day. 



The family of the Celleporidce. includes forms with a calcareous 

 polyzoary, which may be incrusting, or ramose, or massive, and 



Fig. 485. — Retepora cellulosa, from the Red Crag (Pliocene). a, A fragment, of the natural 

 size ; B, Part of the poriferous surface, enlarged. (After Busk — copied from Zittel.) 



which consists of irregularly heaped up zocecia, which are urceolate 

 in form and have sub-terminal mouths. The principal genus in this 

 family is Cellepora itself (fig. 457, b), in which the cell-mouths have 

 in their immediate vicinity one or more ascending rostra carrying 

 avicularia. The genus is widely distributed in the Tertiary rocks, 

 and numerous existing species are known. 



Lastly, we have the singular family of the Selenariidce, including 

 the allied genera Selenaria, Cupularia, and Lunulites (fig. 457, d), 

 in which the polyzoary is unattached, and consists of a plano-con- 

 vex or concavo-convex disc, composed of only one layer of cells, 

 the mouths of which open on the convex surface. The three 

 genera above mentioned are the principal ones comprised in this 

 family, and they range from the Chalk to the present day. 



LITERATURE. 



1. "Catalogue of the Marine Polyzoa in the Collection of the British 



Museum " (Cheilostomata). 1852. Busk. 



2. " Catalogue of the Cyclostomatous Polyzoa in the Collection of the 



British Museum." 1875. Busk. 



3. " Report on the Polyzoa." ' Challenger Reports,' vol. x., 1884, and vol. 



xvii., 1886. Busk. 



4. " Monograph of the Fossil Polyzoa of the Crag." Busk. ' Pakeon- 



tographical Society.' 1859. 



