312 PALEONTOLOGY OF NEW YORK. 



Test thin, marked by fine, concentric striaB, which are fasciculate on the 

 anterior portion of the shell. 



Anterior muscular scar large and not deep, a little anterior to. the beak, 

 with a smaller accessory muscular scar above. Pallial line parallel to the 

 basal margin, terminating in a large, shallow, posterior muscular impression. 

 Interpallial area sometimes pitted. Hinge with an oblique fold in the right 

 valve and a strong ligamental area. 



A large specimen has a length of 72 mm., and a height of 42 mm. Three 

 medium-sized specimens measure respectively 57, 51 and 50 mm. in length, 

 and 34, 30 and 31 mm. in height. 



This species is more elongate than M. ienuistriatus, and is also more convex, 

 with a more prominent umbonal ridge and elevated beaks. Its absolute iden- 

 tity with the genus Microdon has not been fully determined from the internal 

 characters, but its external form and general features ally it more nearly with 

 that type than with any other known genus. 



Formation and localities. In the Hamilton group, at Jefferson, and between 

 Jefferson and Summit, Schoharie county; and below Norwich, Chenango 

 county, N. Y. 



Microdon reservatus. 



PLATE LXXIV, FIGS. 11-13. 



Microdon reservatus. Hall. Pi-elim. Notice Lamellibranchiata, 2, p. 33. 1S70. 



" " *' Pal. N. y., vol. V, i)t. 1. Plates and Explanations: PI. 74, figs. 11-13. 18S3. 



Shell above the medium size, rhomboid-ovate ; length one-fourth greater than 

 the height ; basal margin regularly convex ; posterior margin almost verti- 

 cally truncate. Cardinal line short, arcuate. Anterior end declining some- 

 what abruptly from the beaks and rounded below. 



Valves depressed-convex in the lower part, moderately and regularly con- 

 vex above. 



Beaks at about the anterior third, small, rising little above the hinge-line. 

 Umbonal ridge scarcely defined, extending to the post-basal extremity in a 

 slightly arching direction. 



