244 [February^ 



species, presumptive evidence that the animal of the living shell is generically 

 different from that of the extinct species ? The hinge of a bivalve shell will 

 not always serve to determine a genus, else Platyodon, w^ould be a Mya, and 

 Mycetopus an Anodonta ; but when a material difference exists in the hinge, 

 some important variation may be expected to exist in the animal organization. 

 By means of Comparative Anatomy, Agassiz has discovered differences in the 

 animals of the North American Unios, which he considers sufficient to warrant 

 the construction of several genera, and when these are fully determined and 

 compared with those of Asia, Africa, &c., it will very likely ha found that the 

 various genera are restricted to narrower geographical limits than the mere 

 external characters of the shells would lead us to suppose. 



I have at present not ventured to do more than indicate subgenera^ except 

 among a few distinct groups. In Complanaria, Swai?is., the hinge is so pecu- 

 liar and different from the other genera, that it is fully as well entitled to a 

 generic distinction as Alasmodonta or Barbala. There is another small group 

 which is somewhat isolated, the plicated shells, having bold and distinctive 

 characters. This has been separated from Unio under the name of Pleotomerus. 

 The only aberrant form in the genus is U. crassidetis, Lam., which, however, 

 has no affinity with any other group. JJ. infucatus, though somewhat plicated, 

 has a very different character, and belongs to a distinct group. The Z7. rotun- 

 datu^. Lam., is an isolated shell, with the cardinal teeth more like those of Naia, 

 Swainson, an Asian subgenus, than the other North American Unios. 



REFERENCES. 



Lra. Transactions of the American Philosophical Society, Second Series. 



Synopsis of the Family of Naiades, 1852. 

 Con. Monogjraphy of the Family Unionidae. 



Proceedings and Journal of the Academy of Natural Sciences, Second Series. 



New Fresh Water Shells, May, 1S34. 

 Chentt. Illustrations Conchologiques, &c. 

 CttEMN. Chemnitz (Martini) Neues Syac. Conchyl. 

 Barnes. American Journal of Sciences and Arts, vol. vi, 18. 

 HiLDRETH. lb. ib. ib. xiv. la. 



De5H. Deshayes, New Ed. of Lamarck's Anim. sans Vertebres. 

 Ferussac, in Guerin's Mag. de Zoologie, 1835. 

 L\M Lamarck, Hist. Nat. des Animaux sans Vertebres. 

 Raf Rafinesque, f^es Annales general des Sciences Physiques, 1820. 

 Sat. Nicholson's Encyclopedia, American edition, article (Jonciiology, vol. 4, 1819. 



American Conchologv, with a Synonymy of Western Unios. ' 



Transylvania Journal of Medicine. 



New Harmony Disseminator. 

 Swains. Swainson, Exotic Conchulogy ; Zoological Illustrations; Malacology. 



Note. The Roman numerals refer to the vtdume, and the fieures following indicate the page, 

 pldte and figure;. An asterisk (*; indicates the doubtful species. 



Unio. 



Unio abacus, Hald., Journ. A. N. S. viii. 202. 

 Lea^ Synopsis, 24. 



Aberti, Con., Proceed. A. N. S., March, 1850, v. 10. 

 U. Lamarckianus, Lea, 1852, x. 22, 17, 20. 



acutissimus, Lea, Aug. 1834, iv. 89, 10, IS. 

 Con. Monog. 86,47, 2. 

 Chemt, Conchyl. Unio, pi. 8, fig. 3. 



affinis. Lea, 1852, x. 271, 19, 26, Synopsis 27. 



amoenus. Lea, 1841, viii. 200, 10, 12, Synopsis 29. 

 Chenu, Conchyl. Unio, pi. 33, fig. 5. 



amygdalum, Lea^ 1846, ix. 275, 39, 1. Synopsis 39, 



altilis. Con., New F. W. Shells, 43, 2, 1. 

 Lea, Synopsis, 27. 



