916 PELECYPODA. [Eulamellibranchia. 



valve, of which the right posterior and left anterior are distally sulcate 

 or bifid ; no lateral teeth ; the hinge-plate when developed is usually 

 excavated distally ; there is no circumscribed lunule or escutcheon ; 

 the adductor-scars are subequal, continuous with the pallial line, and 

 close to the hinge-plate ; the margin is entire, the pallial line simple, 

 the pallial area often radiately striate. 



Distribution. Antilles, Brazil, European seas, Red Sea, Indian 

 Ocean, Chinese seas, California, Australasia, &c. 



Fossil. Cretaceous and Tertiary. 



Vernacular Name. Double-tooth. 



Remark. The type is from the Miocene of Italy. 



KEY TO SPECIES. 



A. Shell smooth, with fine growth-lines only. 



a. Shell much inflated, with prominent ventricose beaks, very 



thin . . . . . . . . . . . . globularis. 



b. Shell lenticular, valves moderately convex, beaks but slightly 



elevated, fairly solid . . . . . . . . zelandica. 



B. Shell somewhat irregularly concentrically distinctly ridged, 



globose . . . . . . . . . . . . striata. 



1. Diplodonta globularis, Lamarck, 1818. Plate 58, fig. 19. 



Lucina globularis, Lamarck, A.s.V., v, 1818, 544 ; Hanley, Cat. Rec. Biv. 

 Shells, 77, pi. 14, f. 16 ; Conch. Icon., vi, pi. 9, f. 53 ; Crit. List. 46. 

 Mysia globularis, Lam. : Mutton, C.M.M., 75 ; P.L.S. N.S.W., ix, 525. 

 Diplodonta globularis, Lam. : Hutton, J. de Conch., xxvi, 51 ; M.N.Z.M., 

 156 ; Index, 92 ; Pritchard and Gatliff, P.R.S. Vic., xvii (n.s.), 224. 



Shell orbicular, inflated, with prominent ventricose beaks, inequi- 

 lateral, almost smooth, thin, horny-white. Beaks much inflated, 

 approximate, incurved and directed forward, sharply pointed. 

 Anterior end convex, the dorsal margin horizontal ; posterior end 

 narrowly rounded, the dorsal margin lightly curved, descending ; 

 basal margin regularly rounded. Sculpture consisting of very fine 

 concentric growth-lines of variable strength. Epidermis thin, dull, 

 of horny colour. Colour greyish-brown, the beaks whitish. Interior 

 white, not shining, the interpallial area sometimes faintly punctate. 

 Margins thin and sharp. Hinge-plate narrow, arched ; right valve 

 with a high, pointed, and rather slender anterior, and a strong, broad, 

 bifid cardinal tooth ; left valve with a rather feeble and slightly 

 sulcate anterior and a lamellate, triangularly raised posterior cardinal 

 tooth. Lit/fnuent moderately long, external, the nymphse well deve- 

 loped. Adductor-scars oblong, close to the margin, and continuous 

 with the pallial line, which is simple. 



Diameter Ant.-post., 28 mm. ; dorso-ventral, 25 mm. : thickness, 

 20 mm. 



Type in the Mus. Hist. Nat., Geneva. 



Hob. Stewart Island, in 12-20 fathoms ; Banks Peninsula ; 

 21 J miles north-east of Wreck Reef, in 50-54 fathoms ; off Oamaru, 



