272 
PALAEONTOLOGY OF NEW-YORK. 
have had an external ligamental area. A single cast has, upon the anterior 
portion of the hinge-line, as many as six or seven crenulations; while 
the posterior portion of the cardinal line is too imperfect to show the 
remains of any such characters in that part, had they existed. This fea¬ 
ture of the hinge-line, though of similar character to the Palsearcae, differs 
greatly in the number of teeth, which likewise appear to be simple cre¬ 
nulations transverse to the hinge-line. In this feature, as well as in certain 
external characters, some of these shells resemble Pteroperna ; while 
the Palcearcse, 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 are 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, cor¬ 
responding 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 ex¬ 
panded 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 liinge-structure has not been fully determined, they 
are clearly separable from Ambonychia, taking A.bdlistriata and A.radiata 
