trenton limestone. 163 



Genws AMBONYCHIA. 



[ Greek, afi/3uv, the boss of a shield, and ovu|, a claw or talon ; in allusion to the rounded incurved 



urabenes 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 incurved at the extremity, and equal, or project beyond, the 

 line of the anterior extremity ; surface marked by more or less prominent concentric striae, 

 strong undulations, or fine radiating strise. 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 Inocehamus, and Pterinea, 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 Goldfuss may be referred to the same genus, not being a true Pterinea 

 according to the definition of that author. 



212. 1. AMBONYCHIA BELLISTRIATA (n.fj9.). 



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 striae, 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 becaks, which are very ventricose, and ri.se 

 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* . 



