OF CONCHOLOGY. 145 



mistaken for this shell, but are more elevated, and the apex is 

 erect and nearly central. 



The absence of a gill is a remarkable feature in this family. 

 The most careful dissection revealed none internally, nor is there 

 ingress for water, except through the ovarian duct, anus, or 

 mouth. The edge of the mantle is thick and slightly striate, and 

 if this does not perform the office of a gill, possibly it may be 

 done by the hood, or portion of the mantle immediately over the 

 head and neck. There is quite a large extent of this thin vari- 

 cose membrane, and it is possible that this may be its office. (Fig. 

 3, d.) 



Cryptobranchia alba, Dall, n. s. Plate 15, fig. 3, a, b, c, d. 



Sp. ch. — Shell pure white, smooth, or with extremely faint 

 stri» ; solid ; interior pure white, apex anteriorly directed, in- 

 conspicuous ; shell arcuate before and behind. Length of adult 

 •96, width -70, alt. -40 in'. Number examined, 15 specimens. 



Soft parts as in the last species, but tentacles much smaller, 

 and also the labial tentacles. Rhachidian tooth with longer cusps 

 proportionately than concenirica, somewhat rhomboidal ; base 

 lightly ornate ; anterior corners produced, rounded ; laterals 

 with shafts simple, sinuous, cusps three times (or more) as broad 

 as the shafts, barely denticulate along the edge, and finely striate 

 beneath. Bases of shafts pointed. Number examined, 7 speci- 

 mens. Seniavine Strait, Stimpson. Plover Bay, E. Siberia, 

 Dall. 



This species differs from the last in its smooth shell, greater 

 size, pure whiteness, greater lateral compression, and generally 

 more rounded back, from the less prominent apex. Thetentacula 

 in a specimen twice the size of a concentrica were not half as 

 large. The teeth especially differ in the shape of the central 

 tooth, and the greatly broader cusps of the laterals and their 

 striation, resembling those of Pilidium fuh um (Lo\ en). These 

 stride are what Loven terms "cilia," which misapprehension pro- 

 bably arose from a poor microscope, as those instruments in 1847 

 were very far from their present perfection. 



Cabs. Smithsonian Inst., Bost. Soc. Nat. Hist. 

 ? Cryptobranchia instabilis, Dall, nom. prov. Plate 15, fig. 6. 



iSp. ch. — Shell patelliform, depressed, broad, smooth or lightly 

 striate (young). Apex inconspicuous, in the anterior fourth. 

 Sculpture in the adult only of the concentric lines of growth, 

 which are occasionally impressed. Plane of the base of the shell 

 curved upward anteriorly and posteriorly, without compression of 



