114 CALIFORNIA ACADEMY OF SCIENCES. 



[S. D.] Pecten (Plagioctenium) subventricosus Ball. 



Peclen (Plagioctenium) subventricosus Dall, Trans. Wagner Inst. Sci. , Vol. Ill, Part 4, 1898, 

 p. 707, PI. XXIX, fig. 8. 



"Shell of the type of P. ventricosus Sby., from which it differs by being smaller and less 

 tumid, less expanded laterally, with the ribs rounded, instead of flattened, above, and with narrow 

 interspaces; the tops of the ribs smooth, the sides with a dense fringe of concentriclamellae." (Dall). 



Dimensions. — Long. 65 mm.; alt. 65 mm.; diam. 24 mm. 



Several specimeus of tliis sjiecies were found in the Pliocene of Pacific Beach, 

 near San Diego. 



Pliocene. — Ventura County (Bovver.s): San Diego (Hemphill; Stearns; Arnold). 



35. Pecten (Plagioctenium) ventricosus Sowerby. 



Plate XI, Fics. 3, 3a, 6 and (in. 



PecteJi ventricosus Sby., Thes. Conch., Pecten, 1843, p. 51, PL XII, figs. 18, 19. Gabb, Pal Cal., 

 Vol. II, 1869, p. 104. Cooper, 7th Ann. Rept. Cal. St. Min., 1888, p. 258. ^ P. ticmidus 

 Sby., 1835 (not of Turt., 1S22, nor of Zeiten, 1830) = P. circularis Sby. {ex parte) 

 = P. inca d'Orb., 1847 ^fi'^^ Dall., Trans. Wagner Inst. Sci., Vol. Ill, Part 4, 1S98, 

 p. 710). 



Shell thin, orbicular, with subequal inflated valves which are subequilateral ; uniformly 

 radiately ribbed; about twenty-two strong, angular, even ribs separated by narrow interspaces; the 

 whole surface covered with fine, sharp, concentric, looped lamellce, which exhibits the oblique growth 

 in adult shells; ears moderately small, nearly equilateral in left valve; byssal notch prominent; surface 

 of ears ornamented with fine concentric lamelke of growth, and sometimes with radiating ridges. 



Dimensions. — Long. 59 mm.; alt. 50 mm.; diam. 32 mm.; hinge 31 mm.; angle of dorsal 

 margin 1 10 degrees. 



P. cequisulcaius Carpenter resembles this species very closely, but is thinner 

 and flatter and has narrower ribs. Probably all the Pleistocene forms would come 

 under the head of P. ventr'icosua, but many of them have been labeled P. /.(^qui- 

 sulcatus. Specimens identified by Dr. Dall. 



Figures 6 and 6«, Plate XI, represent a shell which was found in the 

 upper San Pedro series at San Pedro, and which is probably a variety of P. 

 ventricosus. 



Very common in the upper San Pedro series of San Pedro, Long Beach, Los 

 Cerritos, Crawfish George's, and Deadman Island. Found also in Pleistocene on Old 

 Mission ditch, north of Ventura, and in the Pleistocene at Tweuty-sixth Street. Sau 

 Diego. The specimen figured is from the upper San Pedro series at Sau Pedro, and 

 is now in the collection of Delos Arnold. 



Lioing. — Santa Barbara to west tropical America (Dall). 



Pleistocene. — San Pedro; San Diego; Lower California (Hemi'hill; Stearns; 

 Orcutt; and Cooper): San Pedro; Ventura; Sau Diego (Arnold). 



