LOWEK HELDERBERG ROCKS. 273 



as the types, by the strong anterior muscular impression, which does not 

 exist in those shells, and by the numerous teeth ii\ the anterior part of 

 the hinge. 



For these forms I propose the generic name Megambonia. 



Genus Megamboma ( n. g.). 



Shell equivalve or subec^uivalve, 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 tl^e 

 species has been mainly determined by external forn^ and marking and the large 

 anterior muscular scar, which is a conspicuous feature in most of the species. 



IWegaiiiboiiia suborbicularis ( n. s.). 



Platk XLIX. Fio. 4 a, 4 6 = 5 6; and PtATE XLIX A. Fra. 5, 

 Shell depressed-suborbicular ; length and greatest width about equq,l : 

 umbones rounded, moderately elevated, closely incurved ; hinge-Hne 

 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 strise. 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 cast, showing the muscular impression. 

 Fig. 5 6 ( by error for 4 6 on plate). Profile view from the posterior side. 



PLATE XLIX A. 

 Fig. 5. A cast of the left vaWe of the same species. 



Geological position and locality. In tlie shaly limestone of the Lower Helderberg 

 group : Near Carlisle, Schoharie county. 



f PAL.SONTOLOOT III.] 35 



