18 BULLETIN 96, UNITED STATES NATIONAL MUSEUM. 



prominent, smooth, somewhat convex, enlarged on the margins, and 

 much enhirged at the base. The opesium is oval, entire. The ovicell 

 is endozocecial and is a small, smooth, distal convexity. The avic- 

 ularia are very small, straight, salient, elliptical, often provided 

 with a gymnocyst. The ancestrula is surrounded by closed zocecia 

 in which the frontal is perforated by an orbicular pore. 



^ . [7/0=0.20-0.30 mm. 

 Measurements. — (Jpesia , r^^^ r,-,n 



^ [?6)=0.13-0.16 mm. 



[Zs=0.40-0.50 mm (omitting the gymno- 

 Zocecia 1 cyst) . 



[Zs= 0.24-0.80 mm. 



Variations and a.ffi7iities. — The length of the gymnocyst is quite 

 variable even on the same zoarium; therefore in the micrometric 

 measurements it is preferable not to count the gymnocyst for many 

 of the zocecia are devoid of it. The reduction of the zooecial length 

 is frequent in this species and affects the entire zoarium ; it is rather 

 a rare occurrence when some mechanical obstacle is not opposed to 

 the free development of the zocecia. 



Occmrrence. — Vicksburgian : 7^ miles southwest from Bladen 

 Springs, Alabama (very rare). 



Middle Jacksonian: Lenuds Ferry, South Carolina (rare). 



Lower Jacksonian: Jackson, Mississippi (very rare). 



Type.—C2it. No. 62576, U.S.N.M. 



SECTIOM" ni. OVICELL HYPERSTOMIAL, ALWAYS CLOSED BY THE 



OPERCULUM. 



It is not easy to recognize on a fossil form whether the opercular 

 valve does or does not close the hyperstomial ovicell. After many 

 dissections which we have made on living species we have recognized 

 that ovicells of this kind generally leave a concave cicatrix above the 

 mural rim, a part of which is thus concealed. We would add that 

 the different genera grouped in this section, although very natural 

 in themselves, appear to belong to different families which the zoolo- 

 gists alone can determine. 



PERIPOROSELLA, new genus. 



{Peri, around; poros, pores.) 



Each zooecium is surrounded by a special series of dietellae (12-16) 

 communicating with two large septulae. 



Genotype. — Periporosella tantilla, new species. Jacksonian. 



In all the other genera of Membranipores provided with dietellae, 

 the latter occupy only the anterior half of the zooecium. In the 

 genus Periporosella they are, on the contrary, arranged all about the 



