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87 
CLASS GASTROPODA 
immediately below the peripheral keel as in mollusks with a normal aper¬ 
ture. Operculum thin, paucispiral. 
Type in United States National Museum, No. 348531. Length 1.1 mm.; 
greater diameter, 0.9 mm. 
This species differs from the two previously known forms, Amplii- 
thalamus inclusus Carpenter and Amphithalamus tennis Bartsch in being 
much stouter than A. tenuis Bartsch, and in having coarser spiral threads 
on the early whorls, a less strong peripheral keel, and more rounded whorls 
than A . inclusus Carpenter. 
Genus ALVANIA (Leach) Risso, 1820 
Shell oval, turbiniform; spire short, apex sharp; whorls rounded, usu¬ 
ally cancellated; aperture subcircular, crenulated within; outer lip with a 
marginal exterior varix. (Tryon, Structural and Systematic Conchology.) 
Type. Rissoa abyssicola Forbes. 
Distribution. Universally distributed; most abundant in North Tem¬ 
perate zone. 
Alvania castanella Dali, 1886 
Plate 80, fig. 1 
Proceedings of the United States National Museum, 9:307; PI. 3, fig. 5. 
Shell elongate-ovate, thin, yellowish-white. Nuclear whorls about one 
and three-quarters, well-rounded, smooth. Post-nuclear whorls strongly, 
slopingly shouldered at the summit, well-rounded, marked between the 
sutures by six spiral keels, the first of which adjoins the strongly chan¬ 
neled suture, while the second marks the angle of the shoulder; the 
remaining four growing gradually and successively weaker and a little 
more closely spaced. In addition to the spiral sculpture, the whorls are 
marked by numerous very fine, incremental lines. Suture quite strongly 
constricted. Periphery of the last whorl well-rounded. Base moderately 
long, well-rounded, very narrowly umbilicated, marked by six equal and 
equally spaced, spiral keels. Aperture semicircular; posterior angle obtuse; 
outer lip thin, showing the external sculpture within; inner lip thin, 
strongly curved and slightly revolute; parietal wall covered with a moder¬ 
ately thick callus, which renders the peritreme complete; operculum thin, 
horny. Length, 2.4; diameter, 1.3 mm. (Dali.) 
Type in United States National Museum, No. 213677. Type locality, 
Atka Island, Alaska. 
Range. Kyska to Atka Islands. 
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