Bulletin - United States National Museum - Smithsonian Institution

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.
Zocecia 1
.
[Zs=0.40-0.50 mm (omitting the gymnocyst)
[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