Monograjih of the Lamdlihranchicda. 225 



Genus Megamhonm — (Hall, 1859). 



Shell equivalve or subequivalve, inequilateral, subovoid, usually 

 very gibbous in the middle and toward the umbones ; anterior side 

 often lobed or auriculate, a strong muscular impression occupying a 

 considerable portion of this part of the shell ; posterior cardinal margin 

 expanded, more or less compressed and frequently alate ; hinge line 

 crenulated on the anterior end ; teeth numerous. 



Surfoce marked by concentric laraince of growth, and often by fine 

 radiating stria). 



The entire structure of the hinge line is unknown ; and the grouping 

 of the species has been mainly determined by external form and mark- 

 ing and the large anterior muscular scar, which is a conspicuous fea- 

 ture in most of the species. 



Megambonia Jamcsi — (Meek, 1872). 



Shell rather large, a little obliquely subovate in general form, rather 

 convex, the most gibbous part being somewhat above and in front of 

 the middle, more or less abruptly cuneate posteriorly and below ; basal 

 outline regularly rounded ; posterior margin rounding into the base, 

 and ascending with a convex curve and forward inclination to the pos- 

 terior extremity of the hinge, which is not in the slightest degree alate 

 behind ; anterior margin rounding into the base below, and slightly sinu- 

 ous under the lobe-like protuberance or rudimentary wing above, which 

 is convex, slightly more prominent than the margin below, and defined 

 from the swell of the umbonal regions on each side, by an oblique 

 sulcus extending to the hinge margin in front of each beak; hinge 

 equaling about two thirds the anterio-posterior diameter of the valves ; 

 beaks rather prominent, or rising distinctly above the hinge line, but 

 slightly oblique, and distinctly incurved; umbonal slopes broadly 

 rounded ; longer axis of the valves moderately oblique to the hinge 

 line. Surface ornamented by veiy regular, rounded, simple, and de- 

 pressed radiating costee, a little wider than the furrows between, and 

 numbering about five in a space of 0.30 inch, near the middle of the 

 lower margin. 



Height about 2.50 inches; anterio-posterior diameter, 2.16 inches ; 

 qonvexity, 1.50 inch. 



Described from a single specimen — a cast of the exterior, with por- 

 tions of the ventral and anterior ventral margins broken away, and the 

 right beak projecting above the left from disi^lacement. It showed 

 distinct indications of a well defined, moderately wide cardinal area, 

 widest under the beaks, and narrowing to the extremities of the hinge. 



Found at Cincinnati, about 350 feet above low water-mark. 



