302 MEMOIRS OF THE CARNEGIE MUSEUM 
this function spread secondarily to the inner gill. I must confess that this idea 
appears to me as unlikely, since it means a going back to a less specialized stage, 
two organs, each of which originally served a different function, now assuming 
both. The balance of evidence is, I think, in favor of the assumption made here, 
but the question is open to further study. 
3. The Anodontine. Simpson’s genera: Strophitus, Anodonta, Anodontoides, 
Symphynota, and Alasmidonta belong here. The definition of these according to 
shell characters, as given by Simpson, is rather satisfactory, and I merely suggest 
another arrangement of them. There is no question that Strophitus, in spite of 
the peculiarity of the marsupium, comes nearer to Alasmidonta, than to any other 
genus, which is chiefly shown by the heavy sculpture of the beak, and the tendency 
of the inner lamina of the inner gill to become united with the abdominal sac. 
On the other hand Symphynota and Anodonta are more closely allied, as is shown 
by the sculpture of the beak. Anodontoides is essentially an Anodonta, but with 
different sculpture of the beak. We apparently are dealing here with a number of 
types, which have developed independently from a common source, and of which 
each has preserved one or the other primitive character, while it is more advanced 
in other features. Strophitus is extreme in the structure of the marsupium; Ano- 
donta is extreme in the reduction of the hinge-teeth. With regard to the hinge-teeth 
we observe that we have primitive types in Alasmidonta heterodon, and Symphynota 
compressa and viridis; yet they are again peculiarly transformed. The inner lamina 
of the inner gills is more primitive (free) in Anodonta, Anodontoides, and Symphy- 
nota, while a more advanced stage is observed in Alasmidonta and Strophitus. 
Anodonta represents an extreme stage in the development of the supra-anal opening. 
The following sequence possibly would be advisable: Alasmidonta, Strophitus, Sym- 
phynota, Anodontoides, Anodonta. 
4. Lampsiline. The following genera of Simpson’s “Synopsis” belong here: 
Truncilla, Micromya, Lampsilis, Obovaria, Plagiola, Obliquaria, Cyprogenia, Ptycho- 
branchus. Some of them are very well defined, while others stand in need of a 
thorough revision. 
First of all, Ptychobranchus stands by itself, on account of its very unique 
marsupium, the peculiarity of which is fully appreciated by Simpson. Yet, as 
we have seen, Ptychobranchus is truly lampsiline, for it has the characteristic struc- 
ture of the edge of the marsupium. There is no question that it is the most primi- 
tive form in this subfamily, although possibly the present condition of the marsupium 
is not the original one. 
Among the others we see that the structure of the marsupium is practically 
