76 THE NAUTILUS. 



female incident to reproduction, and that Truncilla is the genus in 

 which the sexual differentiation of the female has been carried out 

 to the greatest extent, it would seem desirable that a systematic 

 arrangement of the species in the genus should be, as far as possible, 

 based on that feature. 



At the present time such an arrangement must necessarily be 

 based almost wholly on shell characters, and these are mainly to be 

 found in the position of the so-called marsupial expansion. The in- 

 dications afforded by these characters should, of course, be supple- 

 mented by an exhaustive study of the gill itself and its modifications 

 for the purpose of a marsupium, and, until that is done, it is scarcely 

 advisable to attempt to define the subordinate groups in any formal 

 manner or to change the present classification. 



In respect to their peculiar shell characters, the female Truncilltt 

 fall naturally into three groups : 



1. Those in which the entire post-basal area is occupied by the 

 marsupial expansion. 



This group is more closely allied in this respect to Lampsilis than 

 any of the others, and is apparently the more primitive form. 

 T. perplexa is the leading exponent of this group. 



2. Those in which the marsupial expansion is restricted sub- 

 stantially to an inflation and modification of the posterior ridge. 



This might be considered the next stage of development, and is 

 typified by T. triquetra. 



3. Those in which the marsupial expansion is anterior to the 

 posterior ridge and more or less distinctly separated from it. 



The extreme form of this group is the well-known T. foliata, 

 which may well be considered the most highly organized species of 

 the genus. Between it and T. hayesiana, in which the expansion, 

 though anterior to the posterior ridge, is scarcely differentiated from 

 it, and which may, therefore, be considered the other extreme of the 

 series, there is a very considerable amount of variation in this par- 

 ticular. An intermediate stage is that represented by T. sulcata and 

 lenior, in which the expansion, though distinct, is separated from 

 the posterior ridge by a narrow but deep sulcus, which forms a dis- 

 tinct notch at the margin, instead of the wide emargination that is 

 present in foliata and its immediate allies. 



Arranged in this way the species show a distinct line of evolution, 

 from the simple to the complex, caused by the progressive differ- 



