\'2'2 CAI.IKOUNIA ACADKMY OF SCIKNCKS. 



Periploma ftlaniiiscula Sby. + P. lenticularis Sby. — P. artriiitaria CoN. = ^ /'. alia C. B. Ad. 

 = P. exntnaCvv.. (fide Steakns, Proc. U. S. Nat. M us., Vol. XIll, i8yo, p. 223). 

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



Slidl of medium size, elliptical, inequilateral, inequivalve, fragile; umbones small, anterior 

 to center; posterior e.xtremity long anil evenly rouiuled, but |)roiluced farthest a little above the 

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

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

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

 jj.iirial sinus short, cuneiform. 



Dimensions. — Long. 46 mm.; alt. 2,2) mm.; diam. i.S mm.; umbo to anterior e.xtremity 

 13 mm.; to posterior extremity 33 mm. 



Specimens lilciitifietl by Dr. Dall. 



Rare in the San Pedro series of San I'edro, Los Cerritos, Crawli.sh George's, 

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

 Spanisli Higlit, San Diego. 



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



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



Family XIV. TIIRACIID.E. 



(icnus Thracia (Leach) BlalnoiUe. 



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 jmbesceiis Pult. is a characteristic species. 



46. Thracia trapezoides Cunrad. 



Thracia trapezoides CoN., Wilkes Exped., Vol. X, 1849, p. 723, Pi. XVII, fig. 6. Gabu, Pal. Cal., 

 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 Pedro Pliocene specimen.s 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. Dall labeled these specimens 

 "Thracia? carta." 1\ curta is much less depressed behind the beaks, has a much 



