NORTH AMERICAN EARLY TERTIARY BRYOZOA. 



555 



The mode of calcification of the Adeonidae is more complicated. The primi- 

 tive olocyst secreted by the endocyst is rather thin. It is covered by a pleiimcyst 

 secreted by the endocystal elements which pass through the parietal areolae.and 

 deposit over all of them more or less considerable quantities of lime. When the 

 deposit is somewhat thick the parietal areolae are transformed into true tubules 

 (pi. 15, fig. 14, and text fig. 163H). In transverse thin sections the pleurocystal 

 elements superimposed and oriented in radial fibers (pi. 99, fig. 16) can be clearly 

 seen. 



The frontal avicularium is never visible in the interior; it early has no com- 

 munication with the zooecia, and it is therefore nourished exteriorly by the endocyst 

 proceeding from the areolae and covering the skeleton. 



In tangential thin section these pleurocystal elements are very irregular and 

 are grouped in still more irregular fila- 

 ments (pi. 15, fig. 15). This is the 

 characteristic of this secretion, which is 

 very rare in the Anasca, but which de- 

 velops frequently in the other Asco- 

 phora, and attains its maximum of de- 

 velopment in the Adeonidae. 



Classification. The essential char- 

 acters of classification of the Adeonidae 

 have been given by Waters and Levin- 

 sen. There are three principal groups. 

 The first comprises the species which 



FIG. 165. Genus Meniscopora Gregory, 1893. 

 A. Meniscopora bigibbera Gregory, 1893. Zooe- 



are without frontal ascopores. This is a, X 55. (After Gregory, 1893.) 



the family Meniscoporidae Canu, 1907, B - Mmvmpam simehi Canu, 1907. Views of P er- 



, .... , ,, . istomice and apertura, X 80. (After Canu, 1907.) 



composed of himttistomft. Mi'iinscopora^ , . . 



1 ae, peristomice of zooecium; ai, apertura ol a zooe- 



>CrllZO8tonui, L alvetltUli and brave- pj um ; ge, peristomice of a gonoecium; gi, apertura 



oridf/ia. The second group is composed of a gonoecium. 



of the species which are provided with 



a spiramen and contains the single genus Adeonella. The third group contains 



the species which are provided with frontal ascopores and embraces the genus 



Adeona, if the ascopore is simple and the genus AdeoneUopsis if the ascopore is 



stellate. 



Genus MENISCOPORA Gregory, 1893. 



1893. Meniscopora GREGORY, British Palaeogene Bryozoa, Transactions Zoological Society, 

 London, vol. 13, p. 250. 



The zooecia are trimorphic. The normal axial zooecia have an external aper- 

 ture straighter than that of the marginal zooecia; the aperture is formed of a 

 semilunar anterior and of a very concave posterior part. The gonoecia are larger 

 than the ordinary zooecia and their aperture is of different form. The peristomie 

 is of slight depth. Interzooenal avicularia are rare. Certain lateral areolae are 

 transformed into small frontal avicularia. 



