102 CALIFORNIA ACADEMY OF SCIENCES. 



valve with twenty-eight square topped, rather narrow, radial ridges, separated by shghtly narrower 

 interspaces; cardinal area triangular, broad, faintly sculptured with parallel lines transverse to hinge 

 line; two nearly equal series of teeth, crowded in middle, becoming slightly oblique and further 

 separated at the distal ends; margins serrate. 



Dimensions. — Long. 28 mm.; alt. 26 mm.; distance between dorsal and ventral margins 

 20 mm.; umbo to anterior extremity 11 mm.; to posterior e.xtremity 17 mm. 



'ibis is the only Area so far reported from the Pleistocene of the Pacific Coast. 

 It has been reported by Carpenter as being one of the few species common to both 

 the West Indian and Pacific (Panama) faunas. This species and Hippomjx antiqua- 

 tus, which Carpenter considers equal to H. miirula of the West Indian fauna, are the 

 only two species which occur in the Pleistocene of San Pedro and the living fauna of 

 the West Indies. These two species offer no proof of a close water connection be- 

 tween these two regions during Pleistocene time, for the occurrence in both regions 

 of these two species is merely accidental, or a coincidence. The specimen described 

 was identified by Dr. Dall. 



Rare in the upper San Pedro series of San Petlro; two specimens, a right and 

 a left valve. The specimen figured is from the upper San Pedro serie.< at San Pedro, 

 and is now in the collection of Delos Arnold. 



Lioing. — Gulf of California; Central America; West Indies (Carpenter). 



Pleistocene. — San Pedro (Arnold). 



Superfamily OSTRACEA. 



Family VIII. OSTREID.E. 



Genus Ostrea (Linnf) Lamarck. 



Shell irregular, attached by the left valve; upper valve flat or concave, often plain; lower 

 convex, often plaited or foliaceous, and with a prominent beak; ligamental cavity triangular or 

 elongated; hinge toothless, structure subnacreous, laminated, with a prismatic-cellular substance 

 between the margins of the laminte. 



Type, Ostrea edulis Linne. 



22. Ostrea lurida Carpenter. 



Ostrea lurida Cpr., Brit. Assn. Rept., 1S63, p. 645; Jour, de Conch., Vol. XII, 1865, p. 137. Gabb, 

 Pal. Cal., Vol. II, i86g, p. 106. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 256. Keep, 

 West Coast Shells, p. 164, 1892. Williamson, Proc. U. S. Nat. Mus., Vol. XV, 1892, p. 193. 

 Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1898, p. 687. 



Shell of medium size, irregular, suborbicular, ellipsoidal, or elongated; surface laminated and 

 sometimes irregularly plaited; beak prominent; hinge toothless. 

 Di9netisions. — Long. 35 mm.; alt. 50 mm. 



Grades into two varieties, expansa Cpr., which is nearly circular, and rafoides 

 Cpr., which is of a retldish hue on the interior. Specimen identified by Dr. Dall. 



