596 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. 45. 



white with a pale olive periostracum and a minutely granular surface, 

 the granules being disposed in radial lines with wider interspaces; 

 beaks small, acute, very anterior, prosoccslous, with a minute lunular 

 depression in front of them ; a wide shallow depression with an obscure 

 ridge above it extends backward dorsally to the posterior margin, 

 with a lanceolate area between the ridges of the two valves; base 

 almost parallel with the dorsal hinge line; margins of the valves 

 simple, sharp, slightly tortuous; interior pearly; hinge with a small 

 internal resilium on an inconspicuous nymph, with a very small oval 

 lithodesma. Height 16.5; length 25; length of anterior end 4; 

 diameter, 16 mm. 



In deep water off Mazatlan, Mexico, Cat. No. 266802, U.S.N.M. 

 This is the largest of the genus yet described. 



POROMYA (DERMATOMYA) TENUICONCHA, new species. 



Shell small, thin, olivaceous, the pearly luster showing through the 

 periostracum; equivalve, inequilateral, anterior end shorter, rounded 

 in front; posterior end longer, roundly truncate; beaks prominent, 

 prosocoelous, with a marked but uncircumscribed depression in the 

 lunular region in front of them; interior pearly, brilliant; margins 

 simple, sharp; hinge in the left valve with a small internal resilium 

 seated on an inconspicuous oblique chondrophore, with a notch 

 immediately in front of it, into which fits a projecting denticle on 

 the corresponding part of the opposite valve. Height, 13; length of 

 shell, 16; of anterior portion, 6; diameter, 10 mm. 



In deep water off Monterey Bay, California. Cat. No. 266865, 

 U.S.N.M. 



This recalls P. (D.) cquatorialis Dall, but is more delicate, less 

 inflated and smaller. 



ERYCINA COLPOICA, new species. 



Shell small, white, equivalve, very inequilateral, the anterior end 

 much the longer and somewhat expanded, posterior shorter and 

 smaller, both rounded ; the dorsal and basal margins slightly arcuate, 

 subparallel; surface sculptured only with concentric incremental 

 lines, covered with a thin, pale yellowish periostracum; beaks low, 

 inconspicuous, valves rather compressed; interior polished, hinge^ 



formula ' ' ; chondrophore very narrow, oblique, and posteriorly 



directed. Length of shell, 10; of anterior part, 8; height, 6; diame- 

 ter, 3.5 mm. 



Beach at the head of the Gulf of California. Dr. E. Palmer, Cat. 

 No. 267408, U.S.N.M. 



ROCHEFORTIA COMPRESSA, new species. 



Shell small, thin, glassy, compressed, subovate, inequilateral; 

 beaks low, pointed, inconspicuous; surface very finely, sharply, evenly, 

 concentrically striated; anterior end longer, evenly rounded; poste- 



