144 
MARINE SHELLS OF WEST COAST OF NORTH AMERICA 
of the concentric lines of growth, which are occasionally impressed. Plane 
of the base of the shell curved upward anteriorly and posteriorly, without 
compression of the sides. Shell very thick, solid, muscular impression 
deeply impressed. Long., .56; alt., 20 in. Defl. apex, 120°. (Dali.) 
Type in United States National Museum? Type locality, Sitka, 
Alaska. 
Range. Unalaska to the Shumagins and south to Sitka Sound, Alaska. 
Described as Cryptobranchia instabilis Dali. 
Family ACMAEIDAE 
Genus ACMAEA Eschscholtz, 1830 
Shell conical, patelliform, apex more or less anterior. The shells may 
generally be distinguished from Patella by the different texture and margin 
border of the inside. (Tryon and Pilsbry, Manual of Conchology .) 
Type. Acmaea mitra Eschscholtz. 
Distribution. North Atlantic and Arctic, western coast of North 
America and South America, West Indies, Japan, and Indo-Pacific. 
Acmaea mitra Eschscholtz, 1833 
Plate 85, fig. 5 
Zoological Atlas, 18: PI. 23, fig. 4. 
Testa ovato-rotundata elevata, intus alba, extus lutescente calcarea sca- 
briuscula, vertice subcentrali erecto, obtuso, margine integro. (Esch¬ 
scholtz.) 
Die Schale ist ziemlich dick. Der liingste Durchmesser ihrer Oeffnung 
betragt ungefahr 1 Zoll, und eben so viel betragt auch ihre Hohe-Ihre 
aussere Flache wird von einer massig dicken Lage einer fast Kreideartigen 
aber festen Masse gebildet. An kleinen Exemplaren ist die Oeffnung der 
Schale ovaler, als an den grossern, ja mitunter beinahe elliptisch. (Esch¬ 
scholtz.) 
Shell dull-white, aperture nearly circular, wider behind, in some young 
examples somewhat elongated, oval; form conical, apex erect, nearly cen¬ 
tral, blunt, smooth, posterior surface usually straight, but occasionally a 
little convex; exterior smooth, marked with very faint concentric lines of 
growth, devoid of epidermis; margin entire, polished, with a narrow semi- 
pellucid rim inside. Internally smooth or furnished with grooves radiating 
from the apex more or less strongly marked. Muscular impressions deep, 
strong, horseshoe-shaped, with the marks of the anterior ends of the 
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