BRYOZOAN FAUNA OF VINCENTOWN LIMESAND 43 



rim; the cryptocyst completely surrounds the opesium; it is deep, 

 smooth, convex, more developed proximally. The opesium is an- 

 terior, semielliptical, little elongated, bordered with a thin salient 

 peristome; the proximal lip is more thickened and bears two small 

 opesiular indentations. The ovicell is recumbent, convex, smooth, 

 terminated laterally by two symmetrically arranged horns. The 

 opesium of the ovicelled zooecia is larger, transverse, semielliptical. 

 The zooecial vibracula are placed on the zoarial margin; their 

 frontal, perforated by large pores, is convex and terminated by a 

 salient, oblique mucron partially covering the semicircular opesium. 

 An epizooecial calcification frequently transforms groups of ordi- 

 nary zooecia into kenozooecia, which are perforated simply by a 

 median pore. 

 Measurements. — 



t -r r^r- rsn I h() = 0.12-0.15 111111. 



„ . Zs = 0.5o-0.6 mm. „ . 



Zooecia], _ nQ t UpesmuU _ 0.12 mm (ordinary). 

 \lz U.oo mm. \Lo < nri / . nix 



1 [0.2 mm (ovicelled). 



25 cells in 4 sq. mm. 



Structure. — The genus Ooscinopleura Marsson, 1887, in Europe is 

 found only in the Upper Cretaceous, but in America, where condi- 

 tions were apparent^ more favorable, it continues to the base of the 

 Eocene. We refer the reader to our study of it in 1920 9 for detailed 

 description. 



The structure of the vibracular cells is analogous to that of the 

 vibracula characterizing the Recent family Selenariidae. These are 

 powerful organs whose function has not yet been elucidated. They 

 are always placed on the zoarial margin and often at the bifurcation 

 of two branches. They exercise therefore a zoarial function and are 

 apparently useless in zooecial life. 



The epizooecial calcification, which transforms ordinary zooecia 

 into kenozooecia at the base of many branches, is an extraordinary 

 phenomenon. The process of their formation is always identical. 

 A perforated calcareous lamella begins by closing the opesium of a 

 zooecium; next the cryptocyst is covered and becomes indistinct. 

 Finally, the calcification growing more intense, the zooecium becomes 

 convex and assumes the most diverse polygonal forms. The keno- 

 zooecia are not always basal. They exist frequently even on the 

 edge of the fronds, where they accompany or even replace the 

 vibracula. 



In 1920 we advanced the hypothesis that these zooecia could be 

 hydrostatic, an opinion that has never been criticized but that no 

 longer appears to us to be justified. In marine waters, in reality the 



9 U.S.Nat.Mus.Bull. 106, p. 275, 1920. 

 177635—33— — 4 



