SYNOPSIS OF GENERA. xxix 



Ptychodesma, Hall and Whitfield (Twenty-fourth Ann. Rep. 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 fiiintly marked. Pallial line 

 simple. 



Examples : Nyassa arguta, pi. liii, figs. 7-20. 



Nyassa subalata, pi. liii, figs. 21-26. 



