Bulletin - United States National Museum - Smithsonian Institution

SOUTH AFRICAN MARINE MOLLUSKS. 171
a retractive slant. These ribs become attenuated toward the summit
to which they do not quite extend. They are about one-third
as broad as the spaces that separate them. These spaces are marked
near the periphery by a few, feebly expressed, fine, spiral lirations.
Base sculpture similar to the upper surface except that only a very
small portion of the smooth nuclear whorl is apparent. The ribs here
decrease suddenly in size as they approach the inferior suture, and
vanish just before reaching it. Aperture oblique, subcircular; outer
lip thin, showing the external sculpture within; inner lip decidedly
curved and somewhat reflected; parietal wall covered with a thick
callus.
The type, Cat. No. 250519, U.S.N.M., comes from Port Alfred (Coll.
No. 1392). It shows a little more than the last whorl, and measures:
Altitude, 0.8 mm.; greater diameter, 1.8 mm.
Cat. No. 250518 contains another specimen from the same locality
(Coll. No. 1391).
PONDORBIS, new genus.
Shell minute, dextral, depressed helicoid. Nuclear whorls smooth.
Postnuclear whorls well rounded, ornamented with distantly spaced,
very regular, sublamellar ribs.
Type.— Pondorbis alfredensis.
PONDORBIS ALFREDENSIS, new species.
Plate 36, figs. 1, 2, 3.
Shell minute, depressed helicoid, yellowish white. Nuclear whorls
a little more than one, smooth, well rounded. Postnuclear whorls
well rounded, marked at regularly spaced intervals by very regular,
sublamellar, protractively curved, axial ribs, of which seventeen occur
upon the first whorl, while the one-fifth of a whorl beyond the first
postnuclear whorl contains ten riblets which are less strongly developed,
and more closely spaced. Periphery strongly curved. Base
well rounded, openly umbihcated, showing all the whorls within,
marked like the spire. Aperture circular; peristome complete.
The type, Cat. No. 250557, U.S.N.M., comes from Port Alfred
(Coll. No. 1430). It measures: Altitude, 0.2 mm.; greater diameter,
0.8 mm.
Genus DISCOPbIS de Folin.
DISCOPSIS PLANULATA Sowerby.
Cat. No. 186883, U.S.N.M., contains two specimens from Port
Alfred (Coll. No. 253). Cat. No. 227784, U.S.N.M., three specimens
from the same locality (Coll. No. 879). Cat. No. 227785, U.S.N.M.,
three specimens from the same source (Coll. No. 880).