ara paleontology 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 Palcearcaa, 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 Paloearcae, 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 hinge-structure has not been fully determined, they 

 are clearly separable from Ambonycuia, taking A.bellistriata and A.radiata 



