|2'2 CALIFOKNIA ACADEMY OF SCIENCES. 



Periploma planiiiscula Sby. + P. lenlicularis Sby. = P. argentaria CoN. = P. alta C. B. Ad. 

 = P. excurvaCpR. {fide Stearns, Proc. U. S. Nat. Mus., Vol. XIII, 1890, p. 223).- 

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



Shell of medium size, elliptical, inequilateral, inequivalve, fragile; unibones small, anterior 

 to center; posterior extremity long and evenly rounded, but produced farthest a little above the 

 middle; anterior portion much shorter than posterior, faintly biangulated; surface sculptured by fine 

 concentric, incremental lines; hinge a hollow spoon-shaped process, projecting inwards from below 

 the umbones; this cartilage process is strengthened by an elongated callus slanting anteriorly; 

 pallia! sinus short, cuneiform. 



Ditnensio7is. — Long. 46 mm.; alt. t,t, mm.; diam. 18 mm.; umbo to anterior extremity 

 13 mm.; to posterior extremity 33 mm. 



Specimens identified by Dr. Dull. 



Rare in the San Pedro series of San Pedro, Los Cerritos, Crawfish George's, 

 and Deadman Island. Found also in the Pleistocene at Twenty-sixth Street and 

 Spanish Bight, San Diego. 



Living. — Point Conception south to Mexican coast (Stearns). 



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



Family XIV. THRACIID^. 



Genus Thracia (Leach) Blainville. 



Shell oblong, nearly equivalve, slightly compressed, attenuated and gaping posteriorly; 

 smooth or minutely scabrous; cartilage process thick, not prominent, with a crescentic ossicle; 

 pallial sinus shallow. 



Thracia jmbescens Pult. is a characteristic species. 



46. Thracia trapezoides Conrad. 



Thracia trapezoides CoN., Wilkes Exped., Vol. X, 1849, p. 723, PI. XVII, fig. 6. Gabb, Pal. Gal., 

 Vol. II, 1869, p. 90. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 267. 



Shell of medium size, subtrigonal, compressed, thin ; umbones central, bent slightly posteri- 

 orly ; anterior margin evenly arcuate from umbones, bending off quite evenly into the arcuate 

 ventral margin; anterior extremity most produced near base; posterior dorsal margin depressed near 

 umbo, sloping off slightly concavely to a line which abruptly truncates the posterior extremity ; this 

 truncating line is slightly arcuate, but is angular at both ends; surface sculptured by faint incre- 

 mental lines; a prominent fold runs from the umbones to the lower part of the truncated extremity. 

 Dimensions. — Long. 48 mm.; alt. 38 mm.; diam. 16 mm. 



After comparing the San Peilro Pliocene specimens with several Miocene 

 shells from the Astoria horizon of Blakely, Washington (Conrad's type came from 

 this same horizon), it is evident that the two forms are identical. The San Pedro 

 specimens, however, average much larger in size. Dr. Dull labeled these specimens 

 '■'Thracia? carta." T. curta is much less depressed behind the beaks, has a much 



