12 Assembly 



Bimilar to that of PALiEARCA, differs greatly in the number of teeth, 

 which likewise appear to be simple crenulations transverse to the hinge^ 

 line. In this feature, as well as in certain external characters, some of 

 these shells resemble Pteroperna ; while the Palaearcse, on the other 

 hand, approach in character to Bakewellia. 



Among the fossils of this group are some which, in single valves, 

 present the characters of Avicula, and have heretofore been referred 

 to that genus ; but an examination of specimens which preserve the two 

 valves shows that both valves are gibbous, and that they aro essentially 

 equivalved shells, possessing a more or less conspicuous alation upon the 

 anterior and posterior sides. The surfaces of many of these are marked 

 by strong concentric laminae of growth and fine radiating striae, corre- 

 sponding in this character to the surface of some species of Ambonychia. 



An examination of the casts from the interior of some of these shells 

 shows that they are provided with a very strongly marked muscular 

 impression close to the anterior extremity. On farther comparison, these 

 forms have much resemblance to some in the Upper Helderberg group ; 

 one of which was figured in the Geological Report of the Fourth District 

 in 1843, under the name of Pterinea ? cardiiformis. This species is 

 expanded on the posterior cardinal line, and has an anterior lobe or wing 

 separated by a sinus from the body of the shell, and possessing a very 

 large muscular impression which lies just within this anterior lobe. 



It would appear, therefore, that we have a group of shells, possessing 

 the characters here noticed, and, so far as now known, beginning their 

 existence in the Lower Helderberg group, and extending through the 

 Oriskany sandstone, the Upper Helderberg limestone and the Hamilton 

 group. Although the hinge-structure has not been fully determined, they 

 are clearly separable from Ambonychia, taking A. hellistriata and A. 

 radiata as the types, by the strong anterior muscular impression, which 

 does not exist in those shells, and by numerous teeth in the anterior 

 part of the hinge. 



For these forms I propose the generic name Megamlonia. 



Genus Megambonia.* 



Shell equivalve or subequivalve. inequilateral, subovoid, usually very 

 gibbous in the middle and towards the unbones : 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 ; posterior teeth 

 linear, remote from the umbones. 



* Palaeontology of New-Tork, Vol. in. 



