BIVALVIA. 13] 
positions, but an evident relationship exists between their shelly exteriors, to separate 
which would be a violation to any natural arrangement. 
5. Montacuta? ponactna, 8. Wood. Tab. XI, fig. 3, a—e. 
Montacuta? ponactna. S. Wood. Cat. of Crag Shells, Ann. and Mag. Nat. Hist., 1840. 
—_ ? CYLINDRICA, var. Id. 
Spec. Char. Testa minutd, transversd, donaciformi, compressa, valdé inequilaterali, 
laevigata ; posticé brevissimd, margine dorsali rectiusculd, margine ventrali convewiusculd ; 
cardine edentuld ? foved ligamenti profundd, obliqua. 
Shell small, transverse, compressed, wedge-shaped, very inequilateral, smooth ; 
posterior side very short, dorsal margin on the anterior side, nearly straight, ventral 
margin convex; hinge without teeth? ligamental area oblique and deep, inclining 
backwards. 
Length, {. Height, 2;ths of an inch. 
Var. cylindrica. Length, +th. Height, 2th of an inch. 
Locality. Cor. Crag, Sutton. 
Three or four specimens only of this curious shell have been many years in my 
Cabinet, but nothing like it, either recent or fossil, has fallen under my observation, 
by which it can be even generically associated, although it appears very closely 
allied to this or to the Genus Ke//ia. My two forms therefore are placed here 
provisionally, with the best figures and descriptions their unfavorable condition will 
permit, in order to call the attention of future Collectors to their existence. 
Figure (4) represents the left valve, which is very flat, and very inequilateral ; 
the posterior side being rounded, and scarcely extending beyond the umbo; nearly 
the whole of the shell is on the anterior side, the dorsal margin of which is nearly 
straight, but sloping a little from the umbo, the ventral margin convex, becoming 
narrower at the anterior side, giving a wedge-shaped form to the outline of the 
shell by having a greater height from the ventral margin direct to the umbo; in 
the var. 6. (fig. @) also a left valve, it is not so; the posterior side being there 
rounded, but not higher than any other part of the shell, and having such difference 
only in the outline, I imagine it to be merely a variation, as in all its other characters 
there is a specific resemblance. In an imperfect specimen of the right valve are two 
small prominences of the edge of the shell, one on each side of the ligamental area, 
scarcely deserving the designation of teeth, but perhaps a specimen in a better state 
of preservation might have them more developed. The ligament is wholly internal. 
placed immediately beneath the umbo in an angularly formed pit, inclining on the pos- 
terior side. The exterior is smooth and glossy, with the lines of growth scarcely 
visible. 
