NO. 2183. NEW NORTHWEST AMERICAN BIVALVES— DALL. 415 



basal margin arcuate; sculpture of fine close-set regular, uniform 

 concentric threads over the whole surface; hinge strong, pallial sinus 

 small. Length, 7; height, 4.5; diameter, 2 mm. Cat. No. 151419, 

 U.S.N.M. 



San Pedro, California, Bridwell. 



MACTRELLA CLISIA, Dall, 1915. 



Shell large, white, with a thin, dehiscent periostracum, conspicu- 

 ously arcuate, inflated, and egregiously keeled around the posterior 

 dorsal slope; anterior end produced, attenuated, romided; beaks 

 very high, notably prosocoelous ; posterior slope sharp, posterior end 

 flattened, the flat area bordered with a high keel, in front of which is a 

 marked constriction; the flat area is divided by an elevated ray into 

 two nearly equal parts which are somewhat excavated ; the valve mar- 

 gins near the beaks prominently produced ; sculpture only of lines of 

 growth which are fairly conspicuous; hinge line extremely short, 

 pallial sinus reaching only to the vertical from the posterior lateral 

 teeth, the posterior end with an oval gape. Length, 88; height, 66; 

 diameter, 32 mm. Cat. No. 271481, U.S.N.M. 



Type from Manzanillo, Mexico. Range thence to Santa Elena, 

 Ecuador. 



This remarkable shell is more like the West Indian species than it 

 is to M. exoleta of the Pacific coast, but all its characters are, as it 

 were, exaggerated to a fantastic extent. 



SPHENIA TRUNCULUS, new species. 



Shell short, whitish with a dirty ash-colored periostracum, rude and 

 more or less distorted, abruptly trimcate, almost equilateral, the 

 anterior portion swollen, the posterior part attenuated. Length, 7; 

 height, 4.3; diameter, 4 mm. Cat. No. 160116, U.S.N.M. 



San Diego, California, among barnacles on the wharf piles. 



SPHENIA PHOLADIDEA, new species. 



Shell small, thin, white, with a blackish periostracum, which is con- 

 spicuously laminate on the posterior dorsal area; very inequilateral, 

 the anterior side shorter, the beaks inconspicuous, 4 mm. behind the 

 anterior end; sculpture of rude incremental lines, posterior end 

 abruptly truncate, hardly attenuated; pallial sinus rounded, not 

 reaching the vertical of the beaks; hinge with a prominent toothlike 

 projection in the right valve before, in the left valve behind the resi- 

 lifer. Length, 12; height, 5.3; diameter, 4 mm. Cat. No. 2581, 

 U.S.N.M. 



Santa Barbara, California, Major Rich. 



CORBULA PORCELLA, new species. 



Shell small, ashy white, inequi valve, the left valve smaller; inequi- 

 lateral, the posterior end larger; rounded in front, pointed behind; 



