148 CALIFORNIA ACADEMY OF SCIENCES. 



of growth which are most prominent when projections of the frills; hinge narrow, solid; three 

 cardinal teeth in right valve, two in left; ligament narrow; pallial sinus much shorter than lunule; 

 muscle-impressions distinct. 



Dimensio7is. — Long. 98 mm.; alt. 83 mm.; diam. 62 mm.; lunule 18 mm. 



This is the largest of the genus found in these deposits and is easily 

 distinguishable by its size, prominent concentric frills and numerous radiating ribs. 

 The specimen whose dimensions are given above had twenty-four concentric frills 

 and about ninety radiating ribs. S^jecimens identified by Dr. Dall. 



Rare in the upper San Pedro series of San Pedro, whence the specimen 

 figured came. This specimen is now in the collection of Delos Arnold. 



Living. — -Gulf of California to Panama (Carpenter). 



Pleistocene.- — San Pedro (Arnold). 



84. Venus (Chione) neglecta Soiverby. 



Plate XIV, Fig. 3. 



Venus neglecta Sv.N., Thes. Conch., 1842-1849. Carpenter, Brit. Assn. Rept., 1856, p. 306. 



Shell of medium size and thickness, suboval; surface ornamented with quite numerous, about 

 equidistant, undulating, concentric, low frills, and numerous unequal, rounded, radiating ridges^ 

 which are most prominent on the middle of the shell; lunule prominent, ornamented with minute 

 concentric and radiating sculpture; hinge and pallial sinus are as in C. succinta. 



Dhnensiojis. — Long. 29.5 mm.; alt. 24 mm.; diam. 13 mm ; lunule 6 mm. 



This species is close to C. succincta, but may be differentiated by its more 

 oval outline, thinner shell, relatively less diameter, and less prominent but more 

 numerous concentric frills. Specimen identified as " probably 0. neglecta " by Dr. Dall. 



Rare in ujjper San Pedro series of San Pedro and Los Cerritos. The specimen 

 figured is from the upper San Pedro series at San Pedro, and is now in the collection 

 of Delos Arnold. 



Living. — Gulf of California and Central America (Carpenter). 



Pleistocene. — San Pedro (Arnold). 



85. Venus (Chione) simillima Sowerby. 



Venus simillima Sbv., Thes. Conch., p. 708, PI. CLIII, figs. 17, 18, 1842-1859. 



Chione simillima Sbv., Carpenter, Brit. Assn. Rept., 1863, p. 641. Cooper, 7th Ann. Rept. Cal. 



St. Min., 1888, p. 234. Keep, West Coast Shells, p. 186, 1892. Williamson, Proc. 



U. S. Nat. Mus., Vol. XV, 1892, p. 188. 



Shell of medium size, thick, subcordiform ; surface ornamented by numerous slightly 

 crenulated, refle.xed, concentric frills, which grow closer and closer together as the periphery is 

 approached, and by about twenty bifurcated, radiating ridges, which are largest in the middle of the 

 shell, becoming obsolete anteriorly and posteriorly, and increasing in number, but fading out as the 

 periphery is approached; lunule prominent, ornamented with incremental lamina; hinge narrow; 

 only two prominent cardinal teeth on each valve; pallial sinus very shallow; margin crenulated. 



Dimensions. — Long. 40 mm. ; alt. 35 mm. ; diam. 25 mm. ; lunule 9 mm. 



