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. 1 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. 2 



This was corroborated by Dr. Isaac Lea, 3 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 branchiae. 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 branchise, while in four 

 species Unio multiplicatus Lea, U. rubiginosus Lea, U. subrotundus 

 Lea and U. kleinianus Lea they filled all four leaves of the 

 branchiae. 



In a statement made before the Boston Society of Natural His- 

 tory, 4 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. alattis 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 L. 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 branchiae. 5 



In THE NAUTILUS, 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. Jl. Sci. and Arts, XXVI, 1834, p. 117-120. 



2 Remarks on the vSexes and Habits of some of the Acephalous Bivalve 

 Mollusca. Proc. Am. Assn. Adv. Sci., 1851, p. 85. 



3 Description of New Freshwater and Land Shells. Tr. Am. Phil. Soc. 



4 Proc. Bost. Soc. Nat. Hist., Ill, 1848-51, p. 356. 



5 Ueber die Gattungen unter den Nordamericanischen Najaden. Arch, 

 fiir Naturg., 1852, pp. 41-52. 



