ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 125 



Rare in the lower San Pedro series of Deadmaii Island and upper San Pedro 

 series of San Pedro. Found also at Spanish Bight, San Diego. 



Living. — Straits of Fuca to San Diego (Cooper). 



Pleistocene. — San Pedro (Arnold): San Diego (Arnold). 



Pliocene. — San Benito County (Coojjer). 



Miocene. — Ventura Count}' (Cooper). 



Family XVI. LYONSIID.E. 

 Genus Lyonsia Tiirfon. 



Shell nearly equivalve, left valve largest; thin, subnacreous, close, truncated posteriorly; 

 cartilage plates oblique, covered by an oblong ossicle; pallial sinus obscure, angular. 



Lyonsia norvegica is a characteristic species. 



50. Lyonsia californica Conrad. 



Lyonsia californica Con., Journ. Phil. Acad. Sci., Vol. VII, 1837, p. 248, PI. XIX, fig. 20. Cpr., 

 Proc. Zool. Soc, 1856, p. 210; =L. bracteata Gld. =Z-. iiitida Old., {fide Carpen- 

 ter, Brit. Assn. Rept., 1863, p. 638). Keep, West Coast Shells, p. 202, fig. 174, 1892. 

 Williamson, Proc. U. S. Nat. Mus., Vol. XV, 1892, p. 184. 



Shell of medium size, elongate-oval, truncated and narrowing slightly posteriorly, convex, 

 thin; long, slightly depressed posterior to umbo; short, convex anterior; surface sculptured with 

 fine, concentric, incremental lines; external layer chalky and fugacious, pearly beneath; cartilage- 

 plates oblique, covered by an oblong ossicle; pallial sinus obscure. 



Ditnensions. — Long. 31 mm.; alt. 14 mm.; diam. 11 mm. 



This frail shell is generally found with the outer layer gone, leaving the pearly 

 surface of the lower layer exposed. Outline variable. 



Specimens identified by Dr. Dall. 



Rather common in the Pliocene of Deadman Island, the lower San Pedro 

 series of Deadman Island and San Pedro, and the upper San Pedro series of Dead- 

 man Island, Crawfish George's, and San Pedro. 



Living. — Puget Sound to San Diego (Carpenter). 



Pleistocene. — San Pedro (Arnold). 



Pliocene. — San Pedro (Arnold). 



Genus Mytilimeria Conrad. 



Shell rounded oval, more or less ventricose, equivalve, fragile, covered by a thin caducous 

 epidermis; beaks subspiral; hinge without teeth, but formed of small linear excavations under the 

 beaks to receive the ligament, which contains a small ossicle; muscular impressions small; pallial 

 impression with an obtuse sinus. 



Type, Mytilimeria nvttalli Conrad. 



