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 Praryovon, would be a Mya, and 
Mycertorus an Anoponta; 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 be 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 swbgenera, except 
among a few distinct groups. In Compianaria, Swains., the hinge is so pecu- 
liar and different from the other genera, that it is fully as well entitled to a 
generic distinction as ALasmoponTa or Barsata. 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 PLEcTomERws. 
The only aberrant form in the genus is U. crassidens, Lam., which, however, 
has no affinity with any other group. U. infucatus, though somewhat plicated, 
has a very different character, and belongs toa distinct group. The U. rotun- 
datus, Lam.,is an isolated shell, with the cardinal teeth more like those of Nata, 
Swainson, an Asian subgenus, than the other North American Untos. 
REFERENCES. 
Lx¥a.—Transactions of the American Philosophical Society, Second Series. 
; Synopsis of the Family of Naiades, 1852. 
Con.—Monography of the Family Unionide. 
Proceedings and Journal of the Academy of Natural Sciences, Second Series. 
New Fresh Water Shells, May, 1834. 
CHENU.—illustrations Conchologiques, &c. 
Cuemn.--Chemnitz (Martini) Neues Sysc. Conchyl. 
Barnes.--American Journal of Sciences and Arts, vol. vi. 18. 
HAriLvRETH .~-Ib. ib. ib. xiv. 18. 
DesH.—Deshayes, New Ed. of Lamarck’s Anim. sans Vertebres. 
Ferussac, in Gruerin’s Mag. de Zoologie, 1835. 
T.am.—-Lamarck, Hist. Nat. des Animaux sans Vertebres. 
Rar.—-Rafinesque, f.es Annales general des Sciences Vhysiques, 1820. Be 
Say.—-Nicholson’s Encyclopedia, American edition, article Conchology, vol. 4, 1819. | 
American Conchology, with a Synonymy of Western Unios. 
Transylvania Journal of Medicine. 
New Harmony Disseminator. 
Swains.—-Swainson, Exotic Conchology ; Zoological Illustrations ; Malacology. 
Note. The Roman numerals refer to the volume, and the figures following indicate the page, 
plate 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, 18. 
Con. Monog. 86, 47, 2. 
Chenu, Conchyl. Unio, pl. 8, fig. 3. : 
affinis, Lea, 1852, x. 271, 19, 26, Synopsis 27. 
amenus, Lea, 1841, vill. 200, 10, 12, Synopsis 29. 
Chenu, Conchyl. Unio, pl. 33, fig. 5. 
amygdalum, Lea, 1846, ix. 275, 39,1. Synopsis 39, 
altilis, Cow., New F. W. Shells, 48, 2, 1. 
Lea, Synopsis, 27. 
