86 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
Nuclear whorls one and one-half, well-rounded, marked by about 15 
slender, equal and equally spaced, spiral threads and numerous, very fine, 
axial threads, lending the surface a very minutely pitted appearance. Post- 
nuclear whorls very narrowly subtabulately shouldered at the summit, 
well-rounded, smooth excepting fine incremental lines. Suture moderately 
impressed. Periphery of the last whorl well-rounded. Base long, well- 
rounded, bearing a low, broad, spiral cord on its middle, which bounds the 
posterior termination of the white area. Aperture oval, very oblique; 
peristome double, the inner fusing with the anterior portion of the outer 
lip, while posteriorly it is distinct and at a considerable distance from the 
columella and the parietal wall; the space between the columella parietal 
wall and the inner peristome is bridged over by a concave band of shelly 
matter. Length, 1.1; diameter, 0.7 mm. (Bartsch.) 
Type in United States National Museum, No. 213541. Type locality, 
La Jolla, California. 
Range. Monterey to San Diego, California. 
Amphithalamus stephensae Bartsch, 1927 
Proceedings of the United States National Museum, 70:28; PI. 4, fig. 5. 
Shell minute, pale brown with an ashy tinge except the columellar 
region which is flesh colored; the early whorls when they contain the 
animal are much more dusky. Nuclear whorls 1.5; the first half smooth, 
the rest marked by rather distantly spaced, poorly developed, rather broad 
spiral lirations of which nine are present between the summit and the 
periphery. In addition to this, there are inconspicuous lines of growth. 
Postnuclear whorls strongly rounded, narrowly shouldered at the summit, 
the portion appressed to the preceding turn appearing through the substance 
of the shell as a band. Periphery with a weak keel which is truncated 
rather abruptly posteriorly but grades gently into the substance of the shell 
toward the base. Suture well-marked. Base short, inflated, strongly 
rounded, marked by lines of growth only. There is a heavy callus at the 
insertion of the columella, which at its posterior termination almost forms 
a cord. The columella itself is very heavy and oblique. The conformation 
of the aperture is characteristically Amphithalmid, that is, the aperture 
which is oval is much contracted by having a shelf extending out from the 
columellar and the parietal wall toward the outer lip, contracting the aper¬ 
ture. This shelf forms a decided pit behind its edge. The inner and parietal 
lip of the aperture, therefore, are not in contact with the columellar or the 
parietal wall, but at some distance from it; the posterior portion of the 
outer lip, however, extends upward to the preceding turn, which it joins 
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