120 



BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



are, perhaps, small, simple avicularia without pivot or axis analogous to those in 

 Amphible strum papillatum. 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 Membrendoeciwm an- with the Fareiminariidae. 



MEMBRENDOECIUM TRANSVERSUM, new species. 



Plate 3, figs. 11-13. 



Description. The zoarium incrnsts 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. 



. fAo=0.27-0.30 mm. | /..=.=< U.VO.rin 

 M, inurements. Opesia , Aooecia 



|fo=0.20-0.22 mm, 1/3=0.30-0.3.:. 



mm. 

 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. 



Occurrence Midwayan (Clayton limestone) : Mabelvale, near Little Eock, 

 Arkansas (rare) ; 1 mile west of Fort Gaines, Georgia (rare). 

 C'otypes.Cnt. No. 63791, U.S.N.M. 



MEMBRENDOECIUM DUPLEX, new species. 



Plate 24. figs. 1-6. 



The zoarium incrusts shells (Ostrea). The zooeria ;ire 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 convcxitv. The 



FIG. 29. Genus Membrendoecium Canu and Bassler, 1917. 

 The genotype Membrendoecium papillatum Busk, 1884, 

 X 50. (After Busk.) 



