464 MEMOIRS OF THE CARNEGIE MUSEUM. 



old specimens, while they are lacking in the younger ones, but old specimens also 

 may not show them. Thus these two genera should be compared in this respect 

 with Castalina, and it is quite possible that they have here their roots, the shape 

 of the shell of Castalijia also indicating in its elevated posterior slope the "alate" 

 shape of Hyria. 



Thus we see that the genera Castalina, Castalia, and i)robably also CastalieUa 

 and Callonaia, form a series descended from Diplodon, characterized by peculiar 

 tendencies in the shell and in the soft parts. The shell tends to assume a more 

 or less triangular shape, with high beaks, with a strong posterior ridge, and a trun- 

 cated posterior slope. The hinge-teeth tend to develop vertical ridges, and the 

 interdentum becomes wider. The soft parts tend to close the branchial opening 

 in front. All the rest of the soft parts, however, remain here in a comparatively 

 primitive state, being identical with, or standing very close to, the most primitive 

 type of Diplodon in the interrupted character of the septa of the gills (See below) . 

 It also should be mentioned that the glochidia of these forms, where known, as in 

 Castalina and Castalia, possess the same shape and the peculiar hooks observed in 

 many species of Diplodon, but not in all (See below). 



Apparently a side-branch of this series is formed by Hyria and Prisodon. 

 Here the "alate" character of the shell, slightly indicated in Castalina, is empha- 

 sized, and developed to an extreme degree; the hinge-teeth have a slight tendency 

 to develop vertical ridges, exactly as in Castalina: but in the anatomy these forms 

 do not show the tendency to close the branchial opening in front. Also in the 

 rest of the soft parts they remain upon the primitive Diplodon-stage. 



We may express the affinities as follows : 



Callonaia CastalieUa Prisodon 



Castalia Hyria 



Castalina 



I 

 Diplodon 



The genus Diplodon yet remains to be discussed. We have referred to it 

 above as the most primitive type within the subfamily, and there are undoubtedly 

 a number of species contained in it which have a primitive structure. But there 

 are others which differ from them. The first question, however, to be considered 

 is which characters we should regard as iirimitive. 



As far as the shell is concerned, there is considerable variety within this genus. 



