ARNOLD — THE PALEONTOLOGY AND STRATIGRAPHY OF SAN PEDRO. 99 



Miocene. — Walnut Creek, Contra Costa County; Griswold's, San Benito 

 County; San Juan Capistrano, San Diego County (Cooper). 



Genus Yoldia Moller. 



Shell oblong, slightly attenuated behind; compressed, gaping, smooth or obliquely sculp- 

 tured, with a dark olive, shining epidermis; external ligament slight; cartilage as in Leda; pallial 

 sinus deep. 



Yoldia myalls. Coutli. is a characteristic species. 



17. Yoldia cooperi Gahh. 



Yoldia cooperi Gabb, Proc. Cal. Acad. Sci., Vol. Ill, 1865, p. 189; Pal. Cal., Vol. II, 1869, p. 31, PI. 



IX, fig. 54. Williamson, Proc. U. S. Nat. Mus., Vol. XV, 1892, p. 192. Dall, Trans. 



Wagner Inst. Sci., Vol. Ill, 1898, p. 594. 

 Yoldia inipressa (not of Con., Wilkes Exped., Vol. X, p. 726, PI. XVIII, fig. 13; nor of Meek, 



Smithsonian Check - List, Mioc); in part, Gabb, Pal. Cal., Vol. II, 1869, p. 59; in part, 



Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 270. 



Shell of medium size, oblong, pointed at one end, subcompressed, thin; surface ornamented 

 by numerous small, concentric ribs, abrupt on the upper side, and sloping downward on the side 

 towards the base; beaks minute, placed in advance of middle; anterior end narrow, subacuminate- 

 concave above; posterior end broadly rounded; lunule long, narrow, deeply impressed, smooth; hinge 

 with prominent cup-shaped cartilage-pit, with about twelve sharp teeth in front, and about fifty 

 behind; pallial sinus large, deep and rounded. 



Dimensions. — Long. 64 mm.: alt. 32 mm.; diam. 10.5 mm.; umbo to anterior end 22 mm., 

 to posterior end 44 mm. 



The largest of the Nuculidfe found in these deposits; easily distinguishable by 

 its size, peculiar shape, and teeth. One imperfect specimen measured S3 mm. in 

 length. The Miocene form (Y. impressaj is much smaller and less compressed. 

 Specimens identified by Dr. Dall. 



Rare in upper San Pedro series of San Pedro; three specimens. Found also 

 in Pleistocene of Spanish Bight, San Diego, and Barlow's ranch, Ventura. 



Living. — Santa Cruz to San Diego (Cooper): Half Moon Bay (Arnold). 



Pleistocene. — San Pedro (Cooper; Arnold): San Diego; Ventura (Arnold). 



f?J Pliocene. — San Fernando (Cooper). 



18. Yoldia scissurata Dall. 



Plate XVII, Fig. 13. 



Yo/dia scissurata Dall, Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1898, p. 595. 

 Yoldia arctica Brod. & Sby., Zool. Jour., 1829 (not of Gray, Parry's Voyage App., 1824, fide 

 Dall, Trans. Wagner Inst. Sci., Vol. Ill, 1898, p. 595). 



Shell rather small, oval, compressed, very thin, translucent, only slightly narrowed posteriorly; 

 umbones minute, slightly anterior to middle, the anterior margin is evenly convex; a thin lamina 

 runs along the anterior margin from the umbo to the end of the shell; a much narrower one also 



