450 PROCEEDINGS OF THE ACADEMY OF [1881. 



A. buccula Conr. N. Car. 



Mioc. Foss., p. 60, 



(Not enumerated in the " Smithsonian Check List " of 1864.) 



A. caelata Conr. (IJarbatin). N. Car. 



Mioc. Foss , p. 61. 



A. callipleura Conr. (Scapha'ca). Md. 



Mioc. Foss., p. 54. 



A. Carolinensis Conr. (Noetia). N. Car. 



Proc. A. N. S., 1863, p. 290, as A. ponderosa var. Carolinensis. 



This species differs from the recent A. jDonderoaa (Say) of the 

 Atlantic coast in having a comparatively longer hinge margin, a 

 much more elongated posterior slope, and the umbones less promi- 

 nently incurved. 



A. oentenaria Say (Striarca). Va. ; Md. ; N. Car. 



J. A. N. S., iv, p. 138. Conrad, Mioc. Foss., p. 55. 

 Emmons, Geol. N. Car., 1858, p. 284. 



This species \Qvy closely resembles both in outline and general 

 ornamentation the recent A. solida (Brod. and Swby.), but it 

 lacks the posterior carination of that species, and the ribs are 

 much less distinctly beaded. 



A idonea Conr. (Scapharca). Md. ; N. Car. 



Tert. Fossils, 2d ed., p. 16. Mioc. Foss., p. 55. 

 Emmons, Geol. N. Car., p. 285. 

 A, stilicidium? * Conr. Tert. Foss., 2d ed., p. 15. 



* This species is stated by Conrad (Mioc. Foss., p. 55) to be the young 

 of A. idonea ; I have not seen sufficient specimens of the former to deter- 

 mine this point with positiveness, but the variation scarcely appears to be 

 of specific value. Both species are recorded in the "Smithsonian Check 

 List" for 1864. 



The A. idonea greatl}^ resembles the A. incongrua of Say, a 

 recent species from the southern Atlantic coasts of the United 

 States, from which, however, it can be readily distinguished by 

 several well-defined characters. In A. idonea the anterior ribs 

 are narrower than the interspaces, whereas, in A. incongrua 

 (where the ribs are much more prominently transversely barred) 

 the reverse is very decidedly the case. Again, in A. idonea the 

 hinge area is marked with several " diamond-shaped " longi- 

 tudinal impressions, while in A. incongrua it is transversely 

 striated. The shell in the recent species is also much more 

 prominently inequivalve. 



