364 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



METROPERIELLA BIPLANATA Canu and Bassler, 1917. 



Plate 47, figs. 12-19. 



1917. Metroperiella Uplanata CANU and BASSLER, Synopsis of American Early Tertiary 

 Cheilostome Bryozoa, Bulletin 96, United States National Museum, p. 41, pi. 4, 

 fig. 4. 



Description. The zoarium is free, formed of two flat lamellae, back to back, 

 and inseparable. The zooecia are much elongated, distinct, fusiform: the frontal 

 is convex and formed of a tremocyst with numerous very fine pore?. The aperture 

 is oval, formed of a semilunar anter, and with a wide, rounded rimule. separated 

 by two inner condyles. The ovicell is hyperstomial, large, globular, salient; it 

 completely surrounds the aperture, forming about it a very pronounced peristomie, 

 in which is placed its special orifice; the peristomie is very irregular. The median 

 avicularium is small, little salient, in the immediate vicinity of the rimule. 



fAa=0.16-0.18mm. , fs=1.00-1.10mm. 



Measurements. Aperture \, 1 . Zooecia f 7 - ., - 



Urt=0.14mm. us= 0.50-0.60 mm. 



Variations. As in the preceding species, the passage of the eggs into the 

 ovicell is singularly aided by the development of the special peristomie, in which 

 even the operculum itself is completely concealed when it opens. This operculum 

 can not open without allowing the water to get into the compensation sac. and in 

 consequence without extending its tentacles ; it is therefore rather probable, especially 

 if the larvae are large, that the operculum remains closed during their escape 

 and that the ovicell is indeed closed by a special operculum, as in ScMzopodrella 

 (fig. 15). 



We have observed some avicularian zooecia, as in Metroperiella porosa and 

 Metroperiella grandipora. 



There are some wide zooecia of 0.65 mm. (fig. 12) and some narrow ones of 

 0.40 nun (fig. 16). Very frequently the zooecia are margined (figs. 12, 16): we 

 know that this arrangement has for its object the separation of the ectocyst of the 

 zoarium for a purpose unknown for these rigid species. 



Above many of the zooecia (fig. 16) can be observed a crescent-shaped cicatrix, 

 the morphological significance of which is unknown to us. 



The median avicularium disappears on the ovicelled zooecia. 



The closed zooecia (fig. 15) are, perhaps zooecia which have lost their polypide 

 by accident. 



Affinities. This species differs from MctropericUa porosa, in which the zoarium 

 is 'also bilamellar, in its tremocyst with numerous and very small pores, its small 

 avicularium placed quite near the rimule and its larger zooecial dimensions. 



It differs from Metroperiella grandipora in its bilamellar zoarium, its tremocyst 

 with small pores and its long zooecia. 



Of all the species of Metroperiella this is the only one in which the zooecial 

 length may be 1 mm. 



Occurrence. Middle Jacksonian (Castle Hayne limestone) : Wilmington, North 

 Carolina (very common). 



t. No. 62590, U.S.N.M. 



