124 TEKTIARY AND QUATERNARY PECTENS OF CALIFORNIA. 



rectangularly truncated, and ornamented by more or less prominent radiating- 

 ridges and numerous fine incremental lines. Left valve similar to the right, 

 except that the riVjs are slightly narrower and the interspaces wider: anterior ear 

 showing a shallow concave margin near disk, otherwise ears similar to tliose of 

 the right valve. Hinge similar to that of /•". <-ircul(irlx V8r. mpiixulcatux. 



Dhnensionx. — Alt. 105 n}m. ; long. 112 mm.; hinge line .")6 mm.; diameter 

 8.0 mm. 



This species is close to /*. var. injulsaleatux^ but may be distinguished from 

 that species by its larger size when adiilt, less ventricos'e valves, more and rounder 

 ril)s, and shorter hinge line. The San Fernando forms have slightly squarer ribs 

 and, so far as known, do not reach the enormous size (the type being over 200 

 millimeters in altitude) attained by the specimens from the type locality: otherwise 

 they are identical. One Ventura County specimen lias an altitude of over 100 

 millimeters. 



A variety of this species (No. 140-H: (_". S. ]\1. H.) from Brown's Canyon, 

 near Santa Monica, Los Angeles County, has a somewhat wider umbonal angle 

 (10.5'-) and a slightly ohli(|uoly truncated posterior ear on the right valve: otherwise 

 the specimen is typical. 



/'. oerrosbiisis is confined to the Pliocene, but no doubt ranges through nearly 

 the wholi> of this epoch. The forms in the San Fernando beds show a closer 

 allinity to /'. var. n't/ulsidcaiim, of which they maj' be the precursors. 



Pliocime. Station 3, 1 mile north of Camulos. Ventura County (Watts); San Fer- 

 nando tunnel. Los Angeles County (Ashley. Watts, Arnold): Temescal Canyon, 

 Santa Monica Mountains, Los Angeles County (Rivers): Browns Canyon, Santa 

 Monica, Los Angeles County (Watts); Pacific iicach. well at Twenty-second and 

 H streets, San Diego, and Cholas Valley, San Diego County (Stearns); Pacific 

 Beach (Hemphill, Arnold): Cerros Island, ofi' Lowci- California (^^'atl•h and 

 others); Turth; Hay, Lower California (Hemphill). 



PeCTEN (PlacIOCTEXIUM) (HKIPKKI 11. sp. 



PI. XLIX, tigf. 2, :;, and 4. 



Description. — Shell averaging aliout 30 millimeters in altitude, slightly longer 

 tha^ high, both valves moderately convex, disks slightly obliquely produced 

 posteriorly, moderately thick; disk generally characterized by several more or 

 less prominent concentric zi^ncs of interririited growth, which show as ct)lor bands 

 even on the fossil shells; sides neai'ly straight; margins serrate. Right valve 

 with 18 to 20 flat-topped, squarish ribs, which tend to flatten out and b(>come 

 more convex topped in the later stages of growth; interspaces narrower than 

 the ribs, and crossed on the bottom by numerous, fine, sharp, incremental lirula', 

 which become obsolete on the ribs excepting in the later stages of growth; 

 hinge line somewhat more than one-half length of disk; anterior ear slightly 

 produced and ornamented by \ or 5 prominent radials and numerous incremental 

 lines; byssal notch deep and sharpl}^ defined; posterior ear slightly obliquely trun- 



