18 THE NAUTILUS. 
NOTES ON THE CLASSIFICATION OF THE UNIOS. 
BY CHAS. T. SIMPSON. 
In 1834, Dr. Jared P. Kirtland published the statement that the 
sexes of the North American Unios were distinct, and that the shell 
of the female was characterized by a swelling in the post-basal re- 
gion, which was wanting in that of the male.’ He seems to have 
thought at that time that all the American species were thus dis- 
tinguished, but in a later publication he stated that he believed that 
about two-thirds of the American species have differentiated shells.’ 
This was corroborated by Dr. Isaac Lea,’ who showed that this 
enlargement of the shell of the female was for the purpose of hold- 
ing the charged oviducts, which, in such forms, were found in the 
posterior part of the outer branchiz. Lea, at various times, de- 
scribed the soft parts of some 250 species of Naiades, mostly North 
American Unios, and in a considerable number of these he found 
that the embryos occupied the entire outer branchiz, while in four 
species— Unio multiplicatus Lea, U. rubiginosus Lea, U. subrotundus 
Lea and U. kleinianus Lea—they filled all four leaves of the 
branchie. 
in a statement made before the Boston Society of Natural His- 
tory,’ Agassiz proposed to divide up the Naiades into genera founded 
on the differences of structure of the animal as well as the characters 
of the shell, and to include under one genus a number of species of 
Unios, some of which (including U. alatus Say, the first one in the 
list) have the post-basal inflation of the female shell, and others in 
which it is lacking. Subsequently he used the name Lampsilis, of 
Rafinesque, with Z. cardium Raf. as a type, and he gives in his list 
under this genus a number of species, all of which have the differ- 
entiated shells, and carry the young in the posterior part of the 
outer branchize.° 
In THE Nauriuus, for December, 1895, Dr. V. Sterki published 
the results of his observations on American Unios, and gives some 
1 Observations on the Sexual Characters of the Animals belonging to Lam- 
arck’s family of Naiades. Am. J]. Sci. and Arts, XX VI, 1834, p. 117-120. 
2 Remarks on the Sexes and Habits of some of the Acephalous Bivalve 
Mollusca. Proc. Am. Assn. Adv. Sci., 1851, p. 85. 
§ Description of New Freshwater and Land Shells. Tr. Am. Phil. Soc. 
*Proc. Bost. Soc. Nat. Hist., III, 1848-51, p. 356. 
° Ueber die Gattungen unter den Nordamericanischen Najaden. Arch. 
fiir Naturg., 1852, pp. 41-52. 
