288 MOLLUSCA FROM THE CRAG. 



The form of the shell, and the inequilaterality of the valves are not to be depended 

 upon for specific separation, neither can any reliance be placed upon the two diverging 

 ridges upon the siphonal side, though strongly marked in this, where often these ridges 

 are imbricated; the same character may generally be detected in the younger portion 

 of the preceding, becoming obliterated in the older shell, and what is called the exca- 

 vated lunule,* which is said by the recent conchologists to be the tangible mark of 

 distinction between the two, is as prominent and evident in the one as in the 

 other. 



The remark made by Dr. Gould upon Saxicava distorta may be also applied to our 

 fossils : " it is a perfect Proteus, of which no description can be given that is not 

 liable to mislead." The American shell being probably only a variety of a species 

 that seems to have a geographical extension reaching from the Mgetm to the northern- 

 most shores of Finmark, and the N. E. coast of America. 



Saxicava bilineata, Conrad, ' Foss. of the Mid. Tert. of the United States/ p. 18, 

 PL 10, fig. 4, is probably another variety of this species. 



The only imprisoned specimen I have ever found in the Crag, was in the interior of 

 one of the large barnacles. I have a recent individual from the Bay of Smyrna, it 

 was imbedded in sponge. 



3. SAXTCAVA ? FRAGILIS, Nyst. Tab. XXIX, fig. a e. 



SAXICAVA FRAGILIS. Nyst. Coq. Foss. de Belg., p. 97, pi. 4, fig. 10, a, b, 1844. 



? RUGOSA, juv. ? Forb. and Hani. Hist. Brit. Moll., vol. i, p. 149, pi. 6, figs. 13, 



1848. 



var. Gray. List Brit. Moll., p. 88, 1851. 

 SPHENIA CYLINDRICA. S. Wood. Catalogue, 1840. 



Spec. Char. Testa oblongd, vel snbcylindraced tenui,fragili, inceguilaterali, laevi (prater 

 strias incrementi irregulares] later e postico obtuse angulato ; cardine unidentato. 



Shell oblong, or subcylindrical, thin, fragile, inequilateral, smooth (except the 

 irregular lines of growth) ; posterior side with an obsolete, angular ridge ; one cardinal 

 tooth. 



Length, f inch. Height, f inch. 



Locality. Cor. Crag, Sutton. 



Very abundant. All my specimens were found free and loose in the sand with 

 the valves separated. 



Our shell in its young state is furnished with one cardinal tooth, and a depression 

 for its reception in each valve ; and, like Saxicava, these teeth become obsolete when 



' There is no distinct lunule in these shells, the appearance of such is produced simply by an 

 obsoletely-curved ridge in the shell, on the anterior side. 



