I06 THE AMERICAN NATURALIST. [Vol. XXXVII. 



branchus; but in the latter genus also they are permanently 

 differentiated in the female. 



This fact has possibly not been given clue weight, and Simpson 

 has been severely criticised for giving systematic values to physi- 

 ological features. This character, however, is distinctly an ana- 

 tomical one, and must be regarded as a factor in classification. 



It is well known that in some of the Ouadrula all four bran- 

 chiae bear ova, or embryos, but it has not been proven that this 

 is so in all, especially in younger specimens, such as Q. pustulosis 

 Lea, etc. Neither is it known whether in some species of Unio 

 and Pleurobema the same condition may not be found occasion- 

 ally. Moreover, Simpson himself says that in Anodontoides 

 ferussaciana all branchiae have been found bearing ova, and 

 Gysser makes the same statement in regard to Marfan tana 

 margaritifem. This clearly shows that a distinction between 

 two groups cannot be founded on this feature, the more so, 

 since such closely allied forms as Unio, Pleurobema, and Ouad- 

 rula are separated. 



Differences of Male and Female Shells. — For some groups, 

 these differences have long been known, most of the genera 

 Lampsilis and Truncilla being familiar examples. The distension 

 of the female shell near the posterior-inferior end — as a rule — 

 is obviously the result of the demand for space for the voluminous 

 gravid marsupia near the posterior ends of the outer branchiae. 

 These differences are of various kinds and of very various degrees, 

 as especially among the Truncillae, not only between different 

 groups but also between species of the same groups, and even 

 between different local forms of one and the same species, e. g, 

 Lampsilis luteolus Lam. There are even species where we can 

 speak not only of a distension in the female, but of a different 

 formation of the entire shell, as in Plagiola seeuris Lea. In 

 other forms the differences may be very slight, as in Obov. ellipsis 

 Lea, although its marsupia are of the same type with those of 

 Lampsilis vent ricos its Barnes. The members of the genus 

 Ptychobranchus though having voluminous, and highly differenti- 

 ated marsupia, show no outward differences of the shell, and some 

 females may be even less inflated than males of the same size, 

 as has often been observed by the writer. However, there is a 



