1916 ] Packard: Mesozoic and Cenozoic Mactrinae 285 



Occurrence. — Recent: ley Cape, Alaska, to Neah Bay, Washing- 

 ton (Dall). Pliocene: Wildcat group, Humboldt County, California. 



SPISULA CATILLIFORMIS Conrad 

 Plates 17, 18 and 19 

 Spissula?- catilliformis Conrad, Am. Jour. Conch., vol. 3, p. 193, 1867. 

 Standella California (Conrad), Carpenter, Brit. Assn. Rept. 1863, p. 640 

 (not of Conrad 1837 nor of Deshayes 1852). 



Spistlla catilliformis Conrad, Dall, Nautilus, vol. 7, p. 137, pi. 5, fig. 3, 

 1894; Arnold, Mem. Cal. Acad. Sci., vol. 3, p. 176, pi. 19, fig. 5, 1903. 



Spisula catilliformis, var. alcatrazensis Arnold, Smithsonian Misc. Coll., 

 vol. 50, p. 437, pi. 56, fig. 6, 1907. 



Conrad described this species as follows : 



Suboval, inequilateral; anterior side slightly flattened or contracted; pos- 

 terior side with an oblique shallow groove or fold; lines of growth coarse 

 and prominent; lunule very long, elliptical; ventral margin tumid pos- 

 teriorly; cardinal pit oblique, large; pallial sinus extending beyond the 

 middle of the valve. 



The hinge plate of this species is wide ; chondophore very large 

 and shallow, overhung slightly by the arm of the cardinal and by a 

 prominent spur ; left cardinal heavy, large, situated on the dorsal 

 edge of the hinge plate : arms of nearly equal length ; ventral sinus 

 wide, flat ; right cardinal low, anterior arm small, appressed to the 

 dorsal margin of the shell ; posterior arm thin, overhanging the 

 chondophore ; laterals heavy, anterior lateral short, high and fused 

 to the dorsal side of the anterior cardinal arm ; posterior lateral 

 longer and not quite so high as the anterior one ; laminae very short, 

 heavy, and close to the umbones ; anterior dorsal lamina nearly in 

 the same plane with the anterior cardinal arm. Pallial sinus deep, 

 rounded, reaching nearly to the middle of the shell. The dimensions 

 of the type as given by Conrad are equivalent to a length of 117.3 

 mm., and a height of 98.3 mm. 



There is considerable variation in the outlines and especially in 

 the convexity of the Recent specimens. If the fossil specimens are 

 considered it is found that the variation in shape is quite marked. 

 Those obtained from a single collecting locality from the base of the 

 San Pablo of the Mount Diablo region vary considerably in outline, 

 being more variable than the Recent specimens, although they possess 

 hinges that are nearly identical with those of the Recent fauna. 



The type of the variety alcatrazensis of Arnold was obtained from 



" See foot-note 19. 



