No, 186.J 9 



The Ambonychia obtusa, and an allied fovm from Tennessee, have the 

 same general characteristics ; while there is a distinct external ligamental 

 area, ^'hich is likewise seen in other species. 



A farther examination has shown that some species heretofore referred 

 to the Genus Modiolopsis have this structure of hinge, muscular 

 impressions, etc. ; while some of those from the Trenton limestone, which 

 I referred to Edmondia, have likewise essentially the same structure 

 of hinge as the Ambonychia obtusa. The Edmondia ventricosa, which is 

 farthest removed from the Ambonychia type, has a wide and deep liga- 

 mental area, with three or four oblique teeth beneath the beak, and 

 three lateral oblique teeth at the posterior-extremity of the cardinal line. 



This character of hinge, although so nearly like that of Ambonychia 

 radiata, is nevertheless accompanied by a strong anterior muscular 

 impression, and a less conspicuous posterior one, which renders it neces- 

 sary to separate the two forms. In Edmondia ventricosa, where the 

 hinge-line is less curved than in any other form at present known to 

 me, the structure bears some resemblance to that of Macrodon ; but 

 the cardinal line is never so straight, the posterior teeth are not so 

 nearly parallel with the direction of that line, and the shells are exter- 

 nally marked by concentric striae or laminas, and never by radiating 

 costse as in that genus or in the ark-shells of more recent periods. 



In view of the knowledge we now possess, it becomes necesssary to 

 separate the fossils formerly united under the Genus Ambonychia, and 

 to place those having double muscular impressions under one division ; 

 including with them some forms that have been referred to Modiolopsis 

 and Edmondia. At the same time we are not fully aware of the inter- 

 nal characters of Modiolopsis*; but the typical forms of that genus 

 do not present the exterior features which mark those of the group here 

 noticed, and I shall venture to separate the latter under the name 

 Pal^arca. 



* The type of the Genus Modioloptis is the M. modiolaris ( Cypricardites 

 modiolaris of Conrad = Pterinea modiolaris of the same author, 1838 ) . The 

 Genus Cypricardites, as constituted by Mr. Conrad, embraced species of more 

 than a single genus. The C bisulcata ( 1841 ) is the Pterinea bisulcata of the 

 same author (Report of 1838, p. 116) ; and this fossil is the type of the Genus 

 Grammysia ( G. hamiltonensis ) of De Verneuil. It is probable that a careful 

 investigation of the numerous species of Lamellibranchiata in the Hamilton and 

 Chemung groups will throw some light upon the generic relations of these fossils 

 with those of the lower rocks. An examination of the casts of M. modiolaris, and 

 other allied forms in the Hudson-river group, has not yet disclosed the structure 

 of the hinge ; and the large anterior muscular scar is equally common to those 

 species and many similar forms in the Hamilton group, of which we know nothing 

 of the hinge structure. 



[Assembly No. 186.] 



