350 PALEONTOLOGY OF NEW YORK. 



lamellae and become thickened at their lower extension. The radii are 

 stronger on the posterior part of the shell. 



The hinge structure has the usual characteristics of the genus. 



Four specimens measure respectively 35, 33, 23 and 21 mm. in length, and 

 21, 16, 13 and 10.5 mm. in height. 



This species differs from M. Chemungensis in its wider posterior end, the 

 absence of a constriction in the basal margin, more ovate form and the regular 

 concentric varices. 



Formations and localities. This species occurs in the Hamilton group in most 

 of the localities from Schoharie county to the centre of the State, but it is not 

 very abundant. The greater number have been obtained in Onondaga county 

 and at Canandaigua and Seneca lakes, N. Y. It has been observed in collec- 

 tions from the Hamilton group, near Cumberland, Md. A form undistinguish- 

 able from this species occurs in the soft shales at Bedford, Cuyahoga county, 0. 



Macrodon Chemungensis. 



PLATE LI, FIGS. 11-16. 



Macrodon Cheinungenxix, Hall. Prelim. Notice Lamellibranchiata, 2, p. 14. 1870. 



" " " Pal. N. v., vol. V, pt. 1. Plates and Explanations: PI. 51, figs. 11-16. 



1883. 



Shell of medium size or larger, arcaeforra ; cardinal and basal margins sub- 

 parallel ; length more or less than twice the height ; basal margin nearly 

 straight or gently curving with a slight constriction anterior to the middle. 

 Posterior end broad, sub-truncate or sometimes rounded. Cardinal margin 

 straight, extremities angular. Anterior end rounded below, truncate above. 



Valves convex in the posterior portion, becoming gibbous in the middle 

 and anterior. 



Beaks sub-anterior, prominent, rising a little above the hinge-line. 



Umbonal region gibbous, with a prominent, undefined ridge extending 

 toward the post-inferior extremity, and a vertical depression extending from 

 the umbo to the base. 



Surface marked by irregular, concentric, lamellose lines of growth. 



