1916] Packard: Mesozoic and Cenozoic Mactrinae 287 



This species differs from Spisula catilliformis, with which it is 

 often associated, in its general outline ; in being more ventricose ; in 

 having an indistinct posterior flexure and in the separation of anterior 

 lateral from the anterior arm of the cardinal. 



The type of this species was obtained in the marine cliffs of the 

 Merced group, near Mussel Rock, San Mateo County, California. 



Occurrence. — Pliocene: Merced group, San Mateo County, Cali- 

 fornia. 



SPISULA HEMPHILLII (Dall) 

 Plates 21 and 22 

 Mactra hempMllii Dall, Nautilus, vol. 7, p. 137, pi. 5, fig. 2, 1894. 

 Spisula hemphillii (DalU. Nautilus, vol. 8, p. 40, 1894; Arnold, Mem. 

 Cal. Acad. Sci., vol. 3, p. 175, pi. 19, fig. 3, 1903; Keep, West Coast Shells, 

 p. 106, 1911. 



Dall described this species as follows : 



Shell large, thin, inflated, subequilateral, creamy white with a yellow, 

 thin epidermis, which over the body of .the shell in young shells is beauti- 

 fully evenly concentrically striated and on the posterior dorsal area is irregu- 

 larly wrinkled, with an elevated raphe of epidermis at the margin of the 

 area; beaks rather prominent, the anterior end of the valves longer than 

 the posterior; posterior dorsal slope excavated; lunule obscure, escutcheon 

 marked by prominent elevated radial lines of epidermis; the dorsal margin 

 pouting in front of the ligament, the posterior slope convex, the posterior 

 flexure faint, but marked by a recession of the ventral border of the valves, 

 which gap but very little and not at all in front; anterior end rounded, 

 but smaller than the posterior; ventral border arcuate; hinge and pallial 

 sinus much as in the last species (8. catilliformis), except that the sinus 

 is smaller and less depressed. 



Hinge plate rather wide, chondophore deep, apieally roofed by a 

 prominent spur, but not overhung by the cardinal arms ; left cardi- 

 nal high and heavy, situated on the dorsal portion of the hinge 

 plate ; arms of equal length ; ventral sinus deeply excavated ; right 

 cardinal low, the cardinal arms not fused medially, anterior arm the 

 heavier, posterior arm short and thin ; anterior lateral short, high, 

 round-topped, distinctly separate from and in nearly the same plane 

 with the anterior cardinal arm ; laminae short and heavy ; anterior 

 ventral lamina distant from, but nearly in alignment with the 

 anterior cardinal arm ; dorsal anterior lamina represented by a 

 short, low process arising from the margin of the shell ; posterior 

 dorsal lamina shorter and thinner than the posterior ventral one. 



