ARNOLD — THE PALEONTOLOGY AND STKATIGRAPHY OF SAN PEDRO. 131 



Rather common in the Pliocene; rarer in the lower San Pedro series of Dead- 

 man island; and only found occasionally in the upper San Pedro series of San Pedro, 

 Deadman Island, Los Cerritos, and Crawfish George's. 



Living. — San Francisco to San Diego (Carpenter). 



Pleistocene. — San Pedro (Arnold). 



Pliocene. — Ventura County (Rowers). 



Superfamily LUCINACEA. 

 Family XXI. LUCINID^E. 



Genus Lucina lirugih-e. 



Shell orbicular, white; umbones depressed; lunule distinct; margins smooth or minutely 

 crenulated ; ligament oblique, semi-internal; hinge-teeth two in each valve; laterals, two in right 

 valve, four in left ; muscular impressions rugose ; anterior elongated within the pallial line, posterior 

 oblong; umbonal area with an oblique furrow. 



Lucina jamaicensis Linne is a characteristic species. 



59. Lucina acutilineata Conrad. 



Lucina acutilineata Con., Wilkes Expl. E.xped., Vol. X, p. 725, PI. XVIII, fig. 2, 1849. 



Cyclas -acutiliiicata Coyi.. ]o\iT. Conch., 1865, p. 153. 



Lucina borealis (not of Linn/eus, Syst. Nat., Edition XII, p. 1413); of Cpr., Brit. Assn. Rept., 1863, 

 p. 643; ^= Pectuytcitlus patulus Con., Jour. Conch., 1865, p. 153; not Wilkes Expl. 

 Exped., p. 726. PI. XVIII, fig. 9; ^=Lucina teirica CoN. (yfide Gabb, Pal. Cal., Vol. II, 

 p. 100, 1869). Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 246. 



Shell large, orbicular, only slightly convex, rather thin; umbones depressed, central; surface 

 ornamented by numerous equal, equidistant, sharp, raised, concentric lines; interspaces show lines of 

 growth; lunule small, but deeply impressed and distinct; two sharp cardinal teeth in each valve; 

 lateral teeth nearly obsolete; anterior muscle-impression much elongated. 



Dimensions. — Long. 57 mm.; alt. 53 mm. ; diam. 28 mm. 



This s])ecies is easily distinguishable by its large size and sharp, concentric, 

 rai.«ed lines. This beautiful shell is always found in a fine state of preservation in 

 the Deadman Island formations, in many cases occurring in pairs. The shell is 

 translucent, and in only a few instances have imperfect specimens been found. The 

 convexity of the shell is slightly variable, being more in the younger shells, the older 

 ones being inclined to become flatter. The Pliocene specimens are smaller, more 

 convex, and have the concentric lines much closer set than do the Pleistocene forms. 

 Specimens identified by Dr. Dall. 



Common in Pliocene and lower San Pedro series of Deadman Island and San 

 Pedro; rare in the upper San Pedro series of Deadman Island, Crawfish George's, and 

 San Pedro. Only one or two specimens have been found in each of the upper San 

 Pedro localities; thus practically restricting the northern species to the Pliocene and 



