27 ® 
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 laminae, and never by radiating costae 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 Palhsarca. 
* The type of the Genus Modiolopsis is the M. modiolaris (Cypricardites modiolaris of Conrad = 
Pterinea modiolaris of the same author, 1838). The Genus Cypkicardites, 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. hamilto- 
ner.sis ) of De Verneuil. It is probable that a careful investigation of the numerous species of Lamelli- 
branchiata 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. 
