0TICA SLATE AND HUDSON-RIVER GROUP. 293 



by twenty-five to forty strong simple radii, which are crossed by fine concentric striae ; 

 radii flattened upon the top ; the intermediate spaces are regularly concave grooves, 

 narrower than the radii, and marked by the concentric striae. 



This species has usually been referred to Pterinea carinata of Goldfuss, but it appears 

 to me specifically distinct. The figure of that author, which is larger than figs. 4 a, 6 of 

 our plate, represents the shell as having twenty-three or twenty-four radii, which are 

 proportionally stronger than in this shell, while specimens of equal size with the figure of 

 Goldfuss have from thirty-five to forty radii upon each valve. On this account, principally, 

 I am disposed to consider the succeeding species as identical with that of Goldfuss. 



I regard both this and the following species as differing sufficiently from Pterinea of 

 Goldfuss to be separated from that genus, and to constitute species under the Genus 

 Ambonychia, which is destitute of an anterior wing, while the posterior side is expanded, 

 though scarcely alate, never showing the distinct wing which marks the Avicula and 

 nearly all the species of Pterinea. 



Fig. 4 a, b. The right and left valves of different specimens of this species. 



Fig. 4 e. A smaller individual, with the radii distinctly marked, and more than thirty in number. 



Fig. 4 i. A young specimen, with the radii finely marked. 



Fig. 4 «. A portion of the surface, enlarged. 



Fig. 4 / A cast of the interior of the right valve. The posterior slope shows a straight well defined 



impressed hne. 

 Fig. 4 A. Profile view of a specimen preserving both valves. 

 Fig. 4 i. Lateral view of the same. The difference in form between this and the preceding specimens 



is due to pressure. 

 Fig. 4 k. Enlarged portion of the surface, showing the same structure as 4 «. 

 Fig. 4 _;'. A large specimen, incrusted at its base with a coral, of which an enlarged portion is given 



in the fig. 4 g. 

 Fig. 4 ^ An impression of a part of the valva of a very large and strongly ribbed specimen. 



These specimens, though from different and widely distant localities, have all the same 

 essential characters, and the radii are always smaller and more numerous than the one 

 cited. It differs from the ^. bellistriata and j3. orbicularis of the Trenton limestone, both 

 of which have finer radii and are of different form. 



Position and locality. This is one of the most common fossils of the Hudson-river group, 

 being found throughout the greater part of its thickness, but is unknown in the Trenton 

 limestone or Utica slate. It is abundant at Boonville and Turin, in Lewis county; at 

 Loraine, Jefferson county ; at Pulaski, Washingtonville and Mexico, Oswego county ; near 

 Rome in Oneida county; and I have seen a single specimen from the altered slates near 

 Waterford, Saratoga county. This species is likewise common in many western localities, 

 and I have specimens from Cincinnati and Oxford ( Ohio ) , Madison ( Indiana ), and 

 Maysville ( Kentucky ) . 



