BIVALVIA. 131 



positions, hut an evident relationship exists between their shelly exteriors, to separate 

 which would be a violation to any natural arrangement. 



5. MONTACUTA ? DONACINA, S. Wood. Tab. XL fiff. 3, a c. 



* " O * 



MONTACUTA? DONACINA. S. Wood. Cat. of Crag Shells, Ann. and Mag. Nat. Hist., 1840. 

 ? CYLINDRICA, var. Id. 



Spec. Char. Testa minutd, transversd, donaciformi, compressd, valde inaquilaterali, 

 Icevigatd ; postice brevissimd, margine dor soli rectiusculd, margine ventrali conveximculd ; 

 car dine edentuld ? foved ligamenti profundd, oUiqud. 



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, T \ths of an inch. 



Far. cylindrica. Length, -|th. Height, -^th 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 Kellia. 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 (b) 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. /3. (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. 



