364 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
METROPERIELLA BIPLANATA Cami and Bassler, 1917. 
Plate 47, ligs. 12-19. 
1917. Metroperiella biplanota 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 pores. 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. 
M easurements. — Aperture 
^.$=0.16-0.18 mm. 
?fl! = 0.14 mm. 
Zooecia 
f Lz= 1.00-1.10 mm. 
I Jz= 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 Schizopodrella 
(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 mm (fig. 16). Very frequently the zooecia are margined (figs. 12, 16) ; we 
know that this arrangement lias for its object the separation of the ectocyst of the 
zoarium for a purpose unknown for these rigid species. 
Above many of the zooecia (fie - . 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 Metroperiella 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). 
Cotypes. — Cat. No. 62590, U.S.N.M. 
