BRYOZOAN FAUNA OP VINCENTOWN LIMESAND 85 



The aperture is terminal, semielliptic, lepralif orm ; the proximal 

 lip is straight and bears a small salient mucron transformed rapidly 

 into a true lyrule; the peristome is distal, thin, little salient. The 

 ovicell is hyperstomial not closed by the operculum, widely open, 

 smooth, convex, salient. On each side of the aperture there is a 

 small elliptical avicularium with a pivot, divergent, oblique or trans- 

 verse, oriented proximally. The adult zooecia are indistinct; the 

 frontal is covered by an epicalcification and with one or two small 

 supplementary avicularia; the apertura is deeply embedded at the 

 bottom of the short peristomie formed by the epicalcification; the 

 ovicell is little salient, little visible, surrounded by epicalcification, 

 opening widely into the peristome. The ancestrula is a very small 

 ordinary zooecium without avicularia. Sporadically there is a very 

 large avicularium replacing a zooecium ; it is elliptical, or oval, some- 

 what spatulate, with a pivot. 

 Measurements. — 



rj . \Lz = 0.63-0.72 mm. . \ha= 0.14-0.16 mm. 



Zooecium Apertura 



[lz = 0.29-0.36 mm. [m=0.13 mm. 



. . , . [Lav = 0.63 mm. „ . n \hav = 0.2>0> mm. 

 Avicularium \ Orifice 7 



[lav = 0.31 mm. [lav = 0.18 mm. 



24-26 zooecia in 4 sq. mm. 



Variations. — The variations of this species are considerable, for 

 there is a great difference between the ancestrular zooecia, the young 

 marginal zooecia, and the more numerous adult zooecia. 



The ancestrular zooecia are rarely provided with avicularia ; they 

 bear a pointed mucron and not a lyrule; they are sometimes sur- 

 mounted by a large globular ovicell. 



The young marginal zooecia rarely occupy a large portion of the 

 branches; they are rather regular in their ensemble, but neverthe- 

 less the place and the orientation of the small avicularia are not 

 constant ; the ovicell is less salient than on the ancestrular zooecia. 



On the other zooecia the frontal epicalcification much deranges 

 the place of the primitive avicularia by the addition of pores of 

 calcification and by a row of avicularia. The general aspect is then 

 of a disconcerting irregularity, which recalls that in Beisselina. By 

 noting especially the two more or less deranged primitive avicularia 

 it is easy to discover the new avicularia. 



The function of the zooecial avicularia is difficult to discover 

 because of their irregularity. We suppose it to be the usual one 

 of oxygenation by the renewal of the adjacent water. 



The waters of the Vincentown deposits were certainly calm and 

 rich in diatoms, permitting the rapid and easy development of 

 vigorous species with much calcified zoaria. 



