Braowioropa. 173 
Meristella, Hall. 1859. 
(Plate 34, figs. 1-11.) 
Shells having the same general external characters as Murisra. 
Valves convex, often inflated, cardinal areas obscure. The 
umbo of the pedicle-valve is incurved at maturity, concealing 
most, if not all, of the foramen; in early stages of growth, how- 
ever, the beak is more erect and exposes the deltidial plates in 
an elementary condition of development. The anterior margin 
of the shell is sinuate, and usually there is a sinus on the pedicle- 
valve, with a less conspicuous fold on the brachial valve; some- 
times both valves bear a low sinus, or the sinus on the pedicle- 
valve may be absent, while the fold on the brachial valve is 
present, thus giving the shell a nasute anterior extension ; again, 
fold and sinus may be absent on both valves. 
In the interior of the pedicle-valve the delthyrium is wide, its 
margins being thickened into dental ridges. The teeth are con- 
spicuous, often much thickened and curved backward at their 
tips, interlocking with the opposite valve in such a manner as to 
make a very firm articulation. The teeth are supported by 
lamellz which rest upon the bottom of the valve and are con- 
tinued for a short distance about the posterior margin of the 
muscular impression. In old shells this portion of the valve 
becomes greatly thickened, the muscular impression correspond- 
ingly deepened and the identity of the dental lamellx is 
obscured by their becoming merged with the substance of the 
valve. The pedicle-cavity is deep and frequently shows a strong 
muscular scar. The impression of the diductor muscles is sub- 
quadrate-ovate or subtriangular in outline, very strongly 
impressed and usually clearly divisible into its two lateral com- 
ponents. The central adductor scar is faint, but linear when 
retained. The lateral scars are deeply striated longitudinally. 
The anterior margin of the muscular area is frequently obscure 
but is not infrequently a ridge from which radiate fine, anas- 
tomosing pallial sinuses. In the post-lateral regions the ovarian 
sinuses are sometimes retained. 
In the brachial valve the beak is depressed and sometimes 
obscured by the incurvature of the umbo of the opposite valve. 
The dental sockets are narrow and divergent. The hinge-plate 
is subject to some unessential variation in form. Usually it is 
25 
