104 TERTIARY AND QUATERNARY PECTENS OF CALIFORNIA. 



concave above, lliglit valve somewhat veiitrico.se, and ornamented by 18 to 21 

 strong-, sciuarish, suliequal primary ribs, wliich become more or less dichotomous, 

 and sometimes trichotomous, after 80 or i(> millimeters in length; medial sulcus 

 of rib more or less deep, in some cases being as deep as the interspaces near 

 the ends, thus completely dividing the primary rib; interspaces suTiecpial. much 

 narrower than the ribs, quite deeply channeled, and often ornamented by a small, 

 rounded intercalary riblet; whole surface crossed by numerous line lines of growth; 

 hinge line less than one-half length of disk; anterior ear only slightly longer 

 than left, arcuate in front, and ornamented l)y several obsolete radial ridges and 

 numerous sharp incremental lines; byssal notch quite prominent; posterior ear 

 slightly obli(piely truncated, and ornamented ))y sharp incremental lines and 

 sometimes ))V obsolete radiating ridges. Left valve much compressed; ribs narrow 

 and rounded, more or less sharply toward the top (there being in some cases a 

 narrow, slightly raised line along the top); interspaces wide, and each ornamented 

 by a more or less prominent, rounded, intercalary ri})let; whole surface striated 

 concentrically by tine, sharp, wavy lines; ears obliquely truncated and sculptured 

 similarly to those of the right valve. 



Diiitenxloits. — Alt. 11:^ mm.; long. ll;2 mm.; hinge line .'JOmm. ; diameter IS mm. 



The measurements are of a specimen somewhat l)elow the average in size. A 

 fine specimen from San Gregorio, San Mateo County, measures lSt» mm. in altitude. 



The most variable factors in this species seem to be the depth and width of the 

 medial sulci on the ril)s of the right valve, and the prominence of the intercalary 

 riblet in the left. 



Tlie spei'imcns from San Gregorio and Pescadero. San Mateo County, show, on 

 the average, flatter ribs, niore highly developed medial sulci, and intercalary ribs on 

 the left valve that almost equal the primary ones in importance. Specimens from 

 Purisima and Lobitas, which appear to represent a somewhat lower horizon than the 

 San Gregorio and Pescadero beds, are smaller, slightly more convex, and have two or 

 three less ribs. 



As the forms from both San Diego and San Mateo counties appear to vary 

 through an almost analogous series of mutiitions, the slight difl'erenees enumerated 

 above have not been deemed of even varietal importance. 



A variety of P. healeyi with 17 ribs is found at Brea Canvon, 4 miles southeast 

 of Newhall, Los Angeles County, associated with the following fauna: Phacoldes 

 arutilineatm. P. richthofeni, Doninia ponder osa {1), Tritonluni sp., Oo/ms n. sp. (?), 

 PoDiaulu.r sp., Turr'delJa coopetl, Tellina idse, Neverita recluziana, Trochita flom, 

 Chione near ,mccincta, Cardkiin. near corbis, Nassa near fossata. This fauna is 

 probably well down in the Pliocene. 



P. healeyi appears to have a very limited vertical range at the few localities 

 where it is known. In San Mateo County it is found in the Purisima formation, 

 which lies confoi-inably beneath the Merced series, while in San Diego it is very 

 abundant in beds of lower Pliocene age. The great abundance of this species in both 

 the Purisima and San Diego formations may possibly be taken as evidence of their 

 partial contemporaneity. 



It was with regret that a name so well established in the paleontological litera- 

 ture of the Pacific coast as P. exj?ant<m was found to be preoccupied. Since, however, 



