TRENTON LIMESTONE. 163 
Genus AMBONYCHIA. 
[ Greek, au@uv, the boss of a shield, and ow%, a claw or talon; in allusion to the rounded incurved 
umbones in the typical species.] 
Character. Equivalve, inequilateral, compressed, alate or subalate posteriorly, obtuse 
and abruptly declining or curving downwards on the anterior margin. General form some- 
what obliquely ovate, gibbous or inflated towards the umbones and on the centre of the 
shell; cardinal margin very oblique, or approaching a line parallel to the direction of the 
umbones, which are often incuryed at the extremity, and equal, or project beyond, the 
line of the anterior extremity ; surface marked by more or less prominent concentric striz, 
strong undulations, or fine radiating strie. Muscular impressions large : one in each valve. 
The description includes several species of fossil shells, which, in some respects, vary in 
character, while, so far as regards general form and other prominent features, they evidently 
form a natural group. 
This genus may be again subdivided, when we become better acquainted with the species 
which are at present arranged under it. Several of these species have heretofore been 
referred to InoceRAmus, and Prerinea, to which they bear some resemblance ; but have 
a structure of the hinge different from the former, as well as being equivalve ; while they 
differ from authentic specimens of the latter, in having no anterior alation, which renders 
it desirable to distinguish these early forms by another name. It appears probable that 
Pterinea carinata of Gotpruss may be referred to the same genus, not being a true PreRINEA 
according to the definition of that author. 
212. 1. AMBONYCHIA BELLISTRIATA (nx. sp.). 
Pi, XXXVI. Figs. 4 a, b,c. 
Obliquely subovoid, ventricose, very inequilateral, height much greater than the length ; 
umbones very ventricose, and extended into long and incurved beaks, which bend forward 
at their extremities ; anterior margin not alated, or extending beyond the beaks ; posterior 
side compressed, subalate ; base regularly rounded ; cardinal line short, very oblique to 
the umbones ; surface marked by fine radiating strie, with a few elevated concentric 
lines of growth., 
This beautiful fossil shell is readily distinguished by its external form and markings. 
The umbones are extended into long incurved beaks, which are very ventricose, and rise 
abruptly from the shell towards the summit; the central and lower part of the shell is 
regularly convex, becoming more compressed towards the margin. The anterior side extends 
almost in a right line from the beaks towards the base. 
Fig. 4 a. Left valve of a very symmetrical specimen. 
Fig. 4 b. Right valve of a larger specimen. 
21* 
