LAMELLIBRANCHIA TA. 
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 strise, 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. 
MACRO DON, Lycett. 1845. 
Macrodon Hamiltonle. 
PLATE LI, FIGS. 1-7, 9, 10. 
Macrodon Hamiltoniw, 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 error). 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 liinge-line. Umbonal 
region regularly gibbous. 
Surface marked by regular, sub-equidistant, lamellose, concentric lines and 
by fine radiating strife, which are usually interrupted at the edges of the 
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