EOCENE HORIZONS OF CALIFORNIA 785 



become fine and riearly obsolete on the beaks. From one and 

 one-half miles east of the McCray Wells, Ventura County. Speci- 

 mens in Leland Stanford Junior University Paleontological Col- 

 lection. This is the first species of Isocardia described from the 

 Eocene of California. 



Pinna lewisi n.sp. — -Shell thin, pearly, mytiliform, equivalve, 

 truncate, and wholly open behind, hinge line long; valves triangu- 

 lar, the apical angle being about 45 degrees; convex along the center 

 line and flaring at the margins. Base of shell notched in the middle 

 and convex on either side. Surface marked by five indistinct 

 radiating ribs and concentric lines of growth. From one and one- 

 half miles east of McCray Wells, Ventura County. Specimen in 

 Leland Stanford Junior University Paleontological Collection. 

 Named in honor of Mr. J. O. Lewis of San Francisco. 



lONE FORMATION 



The lone formation is typically represented on the east side 

 of the San Joaquin Valley, extending from a point twenty miles 

 east of Merced to a point forty miles south of Auburn. It is to 

 be correlated with the marine Eocene sediments of Corral Hollow, 

 those in the northern part of the Mt. Diablo Range, and with 

 those on the eastern flank of the St. Helena Range. The forma- 

 tion is characterized by clayey shales and sandstones and by such 

 fauna as Turritella merriami Dickerson^ and Venericardia planicosta 

 var. ionense var. nova, which is similar to V. marylandica Harris^ 

 and V. potopacoensis Harris.^ The formation is probably to be 

 correlated with the Nanjemoy, which, it is thought by Harris, 

 ranges higher than Chickasawan and Upper Lignitic. 



The three horizons each have typical faunas among which the 

 Venericardia and Turritella are represented by very characteristic 

 species. The evolution of Venericardia planicosta Lamarck seems 

 to have been from a square-ribbed variety to one with broad, 

 rounded ribs, and then finally to a smooth form. Conditions were 

 favorable for its more rapid evolution during the Tejon period and 

 a wide range of variation is noted. 



' Bull. Dept. Geo!., Univ. of Calif., Vol. VII, No. 12 (April, 1913), p. 284, PL 13, 

 Fig. 6. 



= Maryland Geo!. Surv., "Eocene," pp. 178-79, PI. XI, Figs. 4-7. ^ Ibid. 



