140 University of California Publications in Geology [VOL. 8 



equal to about one-third the height of the shell; anterior extremity 

 slightly attenuated, evenly rounded; base broadly rounded; posterior 

 evenly rounded, curving gently in the dorsal region to the beak ; sur- 

 face roughened by coarse lines of growth; hinge and interior but 

 imperfectly known. 



Dimensions. Length, 32 mm. ; height, 26 mm. ; diameter of one 

 valve, 5mm. 



Horizon. Lower Martinez, Meretrix dalli zone. 



This form varies considerably from the type specimen to an elon- 

 gated form, which is slightly more ventricose. Gradational types are 

 found between these forms. 



Type from University of California Locality 1556. 



MARTESIA(?), sp. 



Shell elongate with thin test; beaks anterior, incurved, approxi- 

 mate ; posterior dorsal margin concave and undulating ; anterior dorsal 

 margin short, rounded; anterior end rounded; posterior end slightly 

 flaring; ventral margin nearly straight. One beaded umbonal groove 

 diverges from the beak. A posterior groove, which indicates the position 

 of an internal rib, makes a very obtuse angle with the ventral margin. 

 Tube unknown. 



Dimensions. Height, 13 mm. ; length, unknown. 



Occurrence. Found at University of California Locality 784, near 

 Lower Lake. 



The posterior dorsal margin is concave, while that of Turnus plenus 

 Gabb of the Chico is nearly straight. Its flaring posterior end serves 

 to distinguish it from T. plenus Gabb and Martesia clausa Gabb. 



Mr. Barrows, who is making a study of boring molluscs, kindly 

 helped me in determining this form. 



GASTROPODA 



ACMAEA MARTINEZENS1S, n. sp. 

 Plate 12, figure 8 



Shell large, high, elliptical, smooth except for indistinct growth 

 lines; apex rounded and situated two-fifths of length from anterior 

 end ; the slopes on the sides from the apex are much steeper than those 

 to the ends. 



