120 
BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 
are, perhaps, small, simple avicularia without pivot or axis analogous to those in 
Amphiblestrmn papiliatum. On the fossils on account of their simplicity it is 
sometimes almost impossible to distinguish an avicularium from a vibraculum. 
It is evident that the affinities of M embrendoecium, are with the F arciminariidae . 
MEMBRENDOECIUM TRANSVERSUM, new species. 
Plate 3, figs. 11-13. 
Description. — The zoarium incrusts shells. The zooecia are elongated, distinct, 
oval ; the mural rim is prominent, flat, oblique, wrinkled, enlarged at the base. The 
opesium is oval, the narrow end in front, entire or crenulated. The endozooecial 
ovicell is a large, smooth, distal convexity. The avicularium placed in front of 
each zooecium is small, triangular, pointed, with or without pivot, with its axis 
transverse with respect to that of the zooecium. 
Measurements.- 
. \ho— 0.27-0.30 mm. 
-° P e sla {fo = 0.20_0.22 mm, 
Zooecia 
Lz=0. 15-0.50 mm. 
/s=0.30-0.35 mm. 
Variations. — On account of its 
transverse avicularium this species 
forms a rather divergent type in 
the genus; but we have not con- 
sidered it advisable to create a 
special genus for it, because of the 
very restricted number of our 
specimens. In the neighborhood 
of the ancestrula there are re- 
generated zooecia and also closed 
zooecia perforated with a circular 
orifice. The gymnocyst is rarely 
developed. The most striking vari- 
ation observed in the species is the 
strong denticulation of the opesium 
on a specimen from Fort Gaines, 
Georgia. 
O ccun-ence. — Midw ay an (Clayton limestone) : Mabelvale, near Little Rock, 
Arkansas (rare) ; 1 mile west of Fort Gaines, Georgia (rare). 
Cotypes. — Cat. No. 63791, U.S.N.M. 
Fig. 29. — Genus M embrendoecium Canu and Bassler, 1917. 
The genotype M 'embrendoecium papillatum Busk, 1884, 
X 50. (After Busk.) 
MEMBRENDOECIUM DUPLEX, new species. 
Plate 24, figs. 1-6. 
The zoarium incrusts shells ( Ostrea ). The zooecia are elongated, distinct, or 
confluent, oval; the mural rims are distinct, thin, salient, curved, granulated, not 
enlarged at the base, and without dietallae; the mural rims when confluent are 
thick, convex, granulated, with five distal dietellae. The opesium is oval or ellipti- 
cal, very finely denticulated. The ovicell is a small, smooth, distal convexity. The 
