POLYZOA OR BRYOZOA. 429 



pletely calcified. This genus begins at least as early as the 

 Cretaceous period, and is represented by numerous Tertiary 

 and living types. In the family of the Celleporidce, again, we 

 have forms in which the polyzoary is irregular, often massive 

 in its mode of growth, composed of cells which are more or 

 less vertical to its own plane or axis, and which are heaped 

 up or irregularly superimposed upon one another. Cellepora 

 itself (fig. 269, B) is principally Tertiary and Eecent. Another 

 important family is that of the Escharidce, in which the coen- 

 oecium is erect and rigid, with 

 (juincuncially - arranged cells 

 disposed in a single plane on 

 one or both sides of the frond. 

 In Eschara (fig. 270) the 

 polyzoary may be broad and 

 leaf - like, or branched and 

 nearly linear, and the cells Fig> m ._ Eschara RanvilliaiM . Jurassi( , 

 are placed back to back, 



amalgamated with each other by their walls, and opening 

 on both surfaces of the frond. The genus is abundantly 

 represented from the Jurassic to the present day. Bi- 

 flustra (Cretaceous to Recent) resembles Eschara in the 

 composition of the colony out of two layers of cells placed 

 back to back, but the cells are incomplete in front, and pos- 

 sess a raised border, while they are readily separable from 

 each other. A third genus of this family is Retepora 1 , in 

 which the coencecium is more or less branched, often retic- 

 ulate, funnel - shaped, or fan - like,, the cells opening only 

 on one (the upper) surface. The genus is Tertiary and 

 Eecent. 



Still another interesting group is the family of the Selen- 

 ariadce, with the curious allied types of Selenaria, Cupularia, 

 and Lumdites, ranging from the Cretaceous to the present 

 day. In these singular forms (fig. 269, D) the polyzoary is 

 free, and consists of a plano-convex or concavo-convex disc, 



1 As previously remarked, this genus has nothing to do with the forms from 

 the Palaeozoic rocks generally known by the name Retepora. These latter 

 have no claim to this generic title, and should stand under the genus PTiyllo- 

 pora of King. 



