LEPETA. 69 



Differs from Lepeta s. str. in having the apex of the shell more 

 anterior, the surface not granulate, the median cusp of the rhachidian 

 tooth not longer than the side cusps. Dentition of L. concentrica, 

 pi. 40, fig. 35 ; of L. alba, pi. 40, fig. 40. 

 L. CONCENTRICA Midden dorff. PL 40, figs. 33, 34, 35, 36, 37. 



Shell depressed conical, apex directed forward ; front slope one- 

 third the length of the shell or a little less ; surface faintly radiately 

 striate (more distinctly so in young specimens), not decussated or 

 granulose; light-brownish or greenish tinted. 



The outline is ovate, a little narrower in front ; front slope 

 slightly concave, posterior slope convex. The fine thread-like 

 radiating striae are larger on the longer slope of the shell ; they are 

 not interrupted by concentric growth-lines, the latter being incon- 

 spicuous, or sometimes strongly impressed at intervals. Epidermis 

 very thin, yellowish-brown, deciduous. Inside polished, white, the 

 anterior terminations of the muscle-scar a little behind the apex. 

 Edges of shell level, narrowly bordered with gray, especially in the 

 young. Length 20?, breadth 16, alt. 6 mill. 



North Japan, along the Aleutian Is. and along the southern coast 

 of A laska, southward to Puget Sound. 



Patella ( Cryptobranchia) cceca var. concentrica MIDD., Siber. Reise, 

 p. 183, t. 16, f. 6, 1851. Cryptobranchia concentrica DALL, Amer. 

 Journ. Conch, v, p. 143, t. 15, f. 2, 1869; Proc. U. S. Nat. Mus. i, 

 p. 334, 1878. Lepeta cwcoidesf n. sp., CPU., Suppl. Rep. Br. Asso. 

 Adv. Sci. 1863, pp. 603, 651. Lepeta ccecoides COOPER, List Cal. 

 Moll, p. 24. CPR., Proc. Acad. K S. Phila. Apr., 1865, p. 60. 



Differs from L. cceca in the simply striated, not granulose surface, 

 more anterior apex, larger size and more depressed form. 

 Var. INSTABILIS Dall. PL 40, figs. 44, 45, 46. 



Shell depressed, apex anterior, length of front slope contained 

 three and one-half times in the length of the shell ; surface smooth, 

 with occasional rings caused by more impressed growth-lines. 

 Basal margin curved upward at each end. Color whitish. 



The outline is shortly ovate, front and posterior slopes nearly 

 straight, the young may be lightly striate. Shell thick, solid, 

 muscle-impression deep. Length 14, breadth 12, alt. 4 mill. 



Sitka, Alaska, in 10 fms. 



f Cryptobranchia instabilis nom. prov., DALL, Amer. Journ Conch, 

 v, p. 145, t. 15, fig. between 3b and 5b, 1869. C. concentrica v. in- 

 stabilis DALL, Proc. U. S. Nat. Mus. i, p. 335, 1878. 



