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



larger than any specimens of V. planicosta venturensis that 

 have been found. It has been found only in the Tejon forma- 

 tion of the Pacific Coast and is a characteristic horizon marker. 



Introduced here for comparison are figures of V. planicosta 

 ionensis,^'' pi. 11, figs. 1 and 2. This species is from the Umpqua 

 beds, a higher horizon than that of V. planicosta hornii, and 

 represents a later development in the evolutionary series. The 

 young form exhibits the same type of ribs as V. planicosta 

 hornii and at a height of about 10 mm. the ribs are abruptly of 

 a rounded type, while at an elevation of 25 mm. the ribs are 

 obsolete. 



Specimens, probably of this same horizon, have been collected 

 by J. E. Mills from the left bank of Merced River near Merced 

 Falls, Cal., from the upper Tejon. 



Comparing these with eastern species it will be seen that V. 

 planicosta hornii corresponds very well with V. planicosta regia 

 Conrad, of the Aquia and Lower Chickasawan. V. planicosta 

 ionensis and the Venericardia from Merced Falls correspond to 

 a horizon similar to V. planicosta Lam.^* and V. planicosta var. 

 5 from Wood's Bluff ,^* Alabama, of the Nanjemoy, Upper 

 Chickasawan and Lower Claibornian; also to V. marylandica 

 and V. potapacoensis of Harris®"; also V. planicosta lati- 

 cardo^^ of England. 



Since this manuscript was prepared, this subspecies has been 

 described by the writer.®^ The figured specimen with the ribs 

 showing on the umbone is characteristic of the lone formation. 

 The form described by Mr. Dickerson from the Roseburg 

 Quadrangle, Oregon®^ as V. planicosta merriami has not been 

 found in the lone of California. It is possible that the beds at 

 the Oregon locality range a little higher than the lone of Cali- 

 fornia, which accounts for the presence of specimens with 

 totally obsolete ribs. When V. planicosta reaches this extreme 

 stage in its evolution, it may be well to consider it a new 

 species. 



" These specimens were collected by Mr. Harold Hannibal in the Eocene of Wash- 

 ington. 



»s Harris, Bull. 9, Am. Pal., p. 54, pi. 16, figs. 1-4. 



»» Harris, Bull. 9, Am. Pal., p. 54, pi. 16, fig. 5. 



""Mary. Geol. Surv., Eocene, pp. 178-9, pi. 11, figs. 4-7. 



" Ball, Florida Tertiary, vol. 6, pp. 1420-1422. 



"Jour. Geol., vol. 22, No. 8, Nov.-Dec. 12, 1914, p. 785. 



"Cal. Acad. Sci., vol. 4, p. 118, pi. 11, fig. la. Dec. 30, 1914. 



