32 



lamellose ridges, without visible radiatijg striae in the partially 

 exfoliated shell. 



On casts, the large muscular ;mpres*jon is of a circular form, 

 and is situated on the posterior side of the shell at about half 

 its diameter below the hinge line, leaving a distinct depression 

 reaching nearly to the beak, from its advancing with the growth 

 of the shell. V 



This species is easily distinguished Ifrom any other known to 

 me, by its form and lamellose surfac^, which is not strongly 

 ridged as in the A. undata of tie Triton limestone in New 

 York. 



Geological Formation and Lot-ddty. jp. the Trenton limestone 

 group at Mineral Point, and opposite topubuque, in Wisconsin. 

 Collector. T. J. Hale, James Rail, \ 



AMBONYCHIA planiste^ta, (n. s.j 



Description. Shell obliquelv sub-o^jate, with moderately 

 convex valves, most convex a little anterior to the centre ; the 

 anterior side full and rounded, less rentricose posteriorly. 

 Hinge line straight, less than the greatest width of the shell.. 

 Anterior and posterior margins brojdly rounded. Beaks 

 strong, projecting above the hinge and Rightly incurved. 



Surface marked by distant concentric undulations, and by 

 moderately fine radiating striae, which afe flattened and have 

 very narrow interspaces. 



This species differs from the preceding in the less obliquity 

 of the valves in the shorter hinge line! and in the radiating 

 striae. It is more nearly related to A. Orbicularis, of the New 

 York rocks, but is a little more oblique,! less extended anteri- 

 orly, not so ventricose, and the radiating striae are stronger. 



Geological Formation and Locality, iji limestone of the age 

 of the Trenton limestone of New York, at Mineral Point and 

 Beloit, Wisconsin. 



Collector. Prof. Daniels, T. J. Hale. ' 



AMBONYCHIA erecta; (n. s.) 



Description. Shell sub-quadrangular in outline with convex 

 valves, hinge line straight, as long as the greatest width of the 

 shell, forming a little less than a right angle with the anterior 

 border. Posterior slope nearly parallel with the anterior. 

 Basil margin strongly rounded. Beaks in the casts, small, 

 abruptly attenuate, projecting little above the hinge line. 



