442 CALIFORNIA ACADEMY OF SCIENCES [Proc. 4th Ser. 



1906. Mopalia ciliata Arnold, Pectens of Calif., p. 36 (recorded 

 from Pleistocene of San Pedro, Cal.). 



1919. Mopalia mnscosa Chace and Chace, Lorquinia, v. 2, p. 

 42 [2] (recorded from Pleistocene of Pt. Fermin, 

 Cal.). 



Diagnosis: Valves moderately heavy, barely beaked ; lat- 

 eral areas distinct, bounded by a strong rib-like series of more 

 or less overlapping tubercles in front, and a similar but more 

 or less obsolete series of wider tubercles on the sutural margin, 

 the area between closely and finely tubercular ; head valve sim- 

 ilarly sculptured with 8 strongly tuberculose radial ridges in 

 addition to the tubercled thickenings on the sutural margin ; 

 central areas sculptured throughout with numerous low, rather 

 crenulate, longitudinal riblets, sometimes weakly interlatticed 

 by traces of a fine radial liration across the sulci ; sutural lamina? 

 low and broad, the sinus rather shallow ; tegmental border of 

 median valves with a distinct, obtusely rounded, beak-like pro- 

 jection in front; teeth of first 7 valves strongly projecting, 

 those of head valve vertically grooved outside ; tail valve with 

 a single pair of lateral slits and a small angular sinus in the 

 articulamentum which barely indents the tegmental border ; 

 mucro low and strongly posterior ; eaves spongy and crenu- 

 lated; slit formula 8, 1-1, 1-1. 



Recorded Range: 



Pleistocene: Lower San Pedro Formation — Chiton Bed, 

 Point Fermin, California (Chace and Chace, !) : Nob Flill Cut. 

 San Pedro. California (!). 



Upper San Pedro Formation — "Crawfish George's". 

 San Pedro, California ( !) ; "lumber yard", San Pedro, Cali- 

 fornia (Arnold, as ciliata) ; "coal mine". Point Loma, Cali- 

 fornia ( !). 



Formation doubtful — Deadman Island, San Pedro, 

 California ( !). 



Indian middens: Near Cypress Point, Monterey County, 

 California ( !) ; mouth of Topanga Canyon, near Santa Monica. 

 California (Dr. F. C. Clark, Coll., !) ; La Jolla, California ( !). 



Living: Shumagin Islands, Alaska (Dall), to Cedros 

 Island, Lower California; usually between tides. 



