134 STUDIES IN GEOLOGY, No. 3 



similar. Lucina oregonensis Dickerson, 145 from the Tejon 

 Eocene, has more regular concentric sculpture, and is pro- 

 portionally lower. Phacoides m-uirensis Dickerson, 146 from 

 the Martinez Eocene, is closer than oregonensis, but is 

 thicker marginally, has the ventral margin more broadly 

 rounded, and has a posterior sulcus. 



Knowledge of the interior of the shell and the availability 

 of a larger amount of material for study may change the 

 validity of many of these observations, and the author hesi- 

 tated on account of the lack of such evidence to name a 

 species so closely related to other forms as new ; but with 

 these relationships and the possibilities of identity pointed 

 out, future work will probably be made less intricate by 

 the separation of the form instead of including it in a 

 synonymy from which it might necessarily be extricated 

 later. 



Lower Zorritos. Ridge between Ouebradas Heath and 

 Charan. 



Superfamily CARDIACE;A 



Family CARDIIDAE 



Genus CARDIUM (Linnaeus) Lamarck 



Subgenus TRACHYCARDIUM Morch 



Cardium (Trachycardium) zorritensis n. sp. 



PLATE VII Fig. 12. 



Shell thin, inflated, subequilateral. Anterior margin 

 rounded, not produced ; ventral margin rounded ; posterior 

 margin slightly produced. Umbones high, inflated, narrow. 

 Surface generally convex except on either side of the beaks, 

 where a slight concavity leads into the poorly defined areas. 

 Sculpture of 32 smooth A-shaped ribs with V-shaped inter- 



145 Univ. Cal. Bull. Geol., vol. 9, p. 484, pi. 37, figs. la, b, 1916. 

 148 Idem., vol. 8, p. 132, pi. 10, figs, na, b. 1914. 



