SYNOPSIS OF GENERA. 
XXIX 
Ptychodesma, Hall and Whitfield (Twenty-fourth Ann. Hep. N. Y. State 
Museum Nat. Hist., p. 192. 1872). 
[Type, Ptychodesma Knappianum, Hall and Whitfield.] 
Shell equivalve, inequilateral, obliquely sub-ovate. Anterior end short, more 
or less defined by a shallow cincture extending from the beak to the base. 
Posterior extremity broadly rounded. Cardinal line short, nearly straight, 
often arching. Umbonal slope rounded. 
Surface marked by concentric striae of growth. 
Hinge with two or more cardinal teeth and a wide and deeply excavated 
ligamental area, the sides of which are sharply grooved in parallel lines. The 
grooves and ridges are slightly arched beneath the apex of the valves where 
they take their origin. Anterior muscular impression strong. 
Example: Ptychodesma Knappianum , pi. li, figs. 22-27. 
Nyassa, Hall (Preliminary Notice Lamellibranchiata, 2, p. 27. 1870). 
[Type, Nyassa arguta, Hall.] 
Shell equivalve, inequilateral, transversely elongate or sub-elliptical. Beaks 
small, usually appressed, situated near the anterior end. Cardinal line long, 
arcuate. The middle of the shell is often marked by an oblique cincture, 
which produces a constriction in the margin. Umbonal ridge more or less 
defined, extending to the post-inferior extremity. 
Surface marked by concentric lines of growth, and in some species by obscure 
radii. 
The anterior end of the hinge-plate is furnished with a number of irregular 
teeth or plications in the young stages, which become more or less thickened 
and coalescent with age. Posteriorly, the hinge is marked by from one to four 
elongate lateral teeth. Ligament external, inconspicuous. Anterior muscular 
scar strong and deep. Posterior scar larger and faintly marked. Pallial line 
simple. 
Examples: Nyassa arguta, pi. liii, figs. 7-20. 
Nyassa suhalata, pi. liii, figs. 21-26. 
