aro PALAEONTOLOGY OF NEW-YORK. 



sions, 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 ligamental 

 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 im- 

 pression, and a less conspicuous posterior one, which renders it necessary 

 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 struc- 

 ture 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 externally marked by con- 

 centric striae or lamina?, and never by radiating costte as in that genus or 

 in the ark-shells of more recent periods. 



In view of the knowledge we now possess, it becomes necessary 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 internal 

 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 PALiEAKCA. 



• The type of the Genus Modiolopsis is the M. modiolarit (Cypricarditet modiolarU of Cohbad = 

 Pterinea modiolarit of the same author, 1838) . The Genus Ctpbicarditks, as constituted by Mr. Conrad, 

 embraced species of more than a single genus. Tlie C. bisulcata (1841) is the Pterinea biiulcata of the 

 same author ( Report of 1838, p. 116 ) ; and this fossil is tlic type of tlie Genus Grahmysia (6. hamilto- 

 nentW) of De Verneijil. It Is probable that a careful investigation of the numerous species of Lamelli- 

 brancbiata 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 uUied forms 

 in the Hudson-river group, has not yet disclosed tlie structure of the hinge; and tlie large anterior muscular 

 •car is equally common to those species and many siniilur forms iu the Hamilton group, of which wc know 

 notluDg of the binge structure. 



