﻿2 TKUNCILLA 



Genus TRUNCILLA Rafinesquc, 1819. 



Truncilla Rafinesque, J. de Phys., Chimic, Hist. Nat., 

 LXXXVIII, 1819, p. 427. 



Shell rounded or oval, solid, inflated, generally smooth and 

 rayed ; with delicate beak sculpture, which has a tendency to 

 be doubly looped ; that of the female having a very decided 

 inflation in the post-basal region, which is thinner than the 

 rest of the shell, of dififerent texture, often toothed, and usually 

 radiately sculptured ; laterals double in each valve, the inner 

 in the right valve smaller. Animal generally having the inner 

 gill united to the abdominal sac ; female with a heavy flap of 

 the mantle, which fills the post-basal swelling of the shell,, and 

 which has an inner ridge inside at some distance above the 

 edge ; marsupium very distinct, occupying the swollen part of 

 the shell. 



Type: Truncilla triquctcr Rafinesque. 



The species which I have placed in the generic group Trun- 

 cilla though quite devious in form appear to be all rather 

 closely related and to have characters in common, which sep- 

 arate them from all other Naiades. These characters show 

 them to be easily the most highly organized and specialized 

 of all the Unionidie. The most striking character of the 

 group is the remarkable difference between the shells of the 

 males and females. This difference consists principally in the 

 unusual development of what I have called the marsupial 

 swelling of the latter, a character totally wanting in the shells 

 of the males. This is always thinner than the rest of the shell, 

 it is very commonly of somewhat different texture, it is inflated 

 on the disk and usually at the base or posterior base, often 

 into a widely rounded wing. In many cases it is marked off 

 from the rest of the shell by sulci in front or behind or both. 

 It is in nearly all cases more or less radially sculptured and 

 toothed at the edge and the remains of these teeth are seen 

 along the growth lines. The shells of male and female are 

 alike at first and as a rule continue so until thev are from a 



