LOWER HELDERBERG ROCKS. 
273 
as the types, by the strong anterior muscular impression, which does not 
exist in those shells, and by the numerous teeth in the anterior part of 
the hinge. 
For these forms I propose the generic name Megamboniq . 
Genus Megahbonia ( n. g.). 
Shell equi valve or subequivalve, inequilateral, subovoid, usually very 
gibbous in the middle and towards the umbones : anterior side often 
lobed or auriculate, a strong muscular impression occupying a con¬ 
siderable 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. 
Surface marked by concentric laminae of growth, and often by fine 
radiating striae. 
The entire structure of the hinge-line is unknown; and the grouping of the 
species has been mainly determined by external form and marking and the large 
anterior muscular scar, which is a conspicuous feature in most of the species. 
Megambonia suborbiculails (n. s.). 
Plate XLIX. Fig. 4 a , 4 b — 5 b ; and Plate XLIX A. Fig. 5. 
Shell depressed-suborbicular; length and greatest width about equal : 
umbones rounded, moderately elevated, closely incurved; hinge-line 
less than the greatest width of the shell : anterior muscular impres¬ 
sions large, moderately deep. 
The specimens examined are casts, one of which preserves the form entire, 
showing some remains of concentric striae. The muscular impressions are close to 
the anterior margin, which is compressed from a little distance below, 
Fig. 4 a . The left side of the east, showing the muscular impression. 
Fig. 5 b (by error for 4 b on plate). Profile view from the posterior side. 
PLATE XLIX A. 
Fig. 5. A east of the left valve of the same species. 
Geological position and locality. In the shaly limestone of the Lower ITelderberg 
group : Near Carlisle, Schoharie county, 
f Paleontology III.] 35 
