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 Savicava 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 bemg probably only a variety of a species 
that seems to have a geographical extension reaching from the Algean to the northern- 
most shores of Finmark, and the N. E. coast of America. 
Savicava 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. SAXICAVA? FRAGILIS, Vyst. Tab. XXIX, fig. a—e. 
SaXICAVA FRAGILIS. Nyst. Coq. Foss. de Belg., p. 97, pl. 4, fig. 10, a, 6, 1844. 
7 — RuGOSA, juy.? Ford. and Hani. Hist. Brit. Moll., vol. i, p. 149, pl. 6, figs. 1—3, 
1848. 
— — var. Gray. List Brit. Moll., p. 88, 1851. 
SPHENIA cyLINnDRIca. S. Wood. Catalogue, 1840. 
Spec. Char. Testa oblongd, vel subcylindraced tenut, fragili, inequilaterali, levi (preter 
strias incrementi irregulares) latere 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, 2 inch. Height, % 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 Sazicava, these teeth become obsolete when 
* There is no distinct lunule in these shells, the appearance of such is produced simply by an 
obsoletely-curyed ridge in the shell, on the anterior side. 
