LAMELLIBRANCHIATA. 349 



Valves convex below, rising rather abruptly to the middle where they are 

 gibbous or ventricose. 



Beaks sub-anterior, prominent and incurved. Umbonal slope regularly 

 rounded, and the convexity is continued to the posterior extremity. Below 

 the umbonal slope is a broad constriction occupying one-third of the length 

 of the shell. 



Surface marked by fine concentric striae, and on the anterior and basal 

 portions by strong concentric undulations. 



The specimen described has a length of 56 mm. and a height of 34 mm. 



This form is larger than any other known species of the genus. The con- 

 striction near the posterior end is broader than, in any other. Its absolute 

 identity with Palseoneilo has not been determined. 



Formation and locality. In the Corniferous limestone, at Delaware, Ohio. 



MACRODON, Lycett. 1845. 

 Macrodon Hamiltoni^. 



PLATE LI, FIGS. 1-7, 9, 10. 



Macrodon Hamiltonite, Hall. Prelim. Notice Lamellibranchiata, 2, p. 13. 1870. 



» <• •' Pal. N. Y., vol. V, pt. 1. Plates and Explanations: PI. 51, figs. 1-10 (fig. 



8 in en-or). 1883. 



Shell of medium size or larger, sub-elliptical or sub-ovate, wider behind ; length 

 about twice the height; basal margin broadly curving, sometimes nearly 

 straight in the anterior portion. Posterior extremity broadly rounded, some- 

 times sub-truncate in the upper half Cardinal line essentially straight, 

 obtusely sub-angular at both extremities. Anterior end abruptly rounded or 



sub-truncate. 



Valves convex in the posterior portion and gibbous in the anterior and 



umbonal portions. 



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



region regularly gibbous. 



Surface marked by regular, sub-equidistant, lamellose, concentric lines and 

 by fine radiating strise, which are usually interrupted at the edges of the 

 45 



