20 NAIADES OF MISSOURI 



are so inconstant that this subfamily may be termed a great 

 group of intergrades. Having very typical hinge teeth and very 

 closely adhering valves the branchial margins are not well papil- 

 losed and the soft parts of the different species are more or less 

 identical. In contrast with the other more modern subfamilies, 

 Anodontinae and Lampsilinae, a greater differentiation of soft 

 parts is noted in the latter, due to their more gaping valves 

 and to a greater adjustment to aeration of the embryos; then, 

 too, Unioninae differs from either of the two in that its breeding 

 season is short (tachytictic), being confined to the summer. In 

 the Unionae the color, form and solidity of the conglutinates can 

 be considered as of greater systematic value than in the other 

 sub-groups. It is to be noted that these summer breeders have 

 the peculiar trait of aborting their conglutinates when they may 

 be disturbed from their natural beds. The fact of the close, 

 or even deciduous, mantle connection between the anal and the 

 supra-anal openings may be a minor character in distinguishing 

 the genera. The connection between inner laminae of the inner 

 gills and the visceral mass may also serve in making distinction. 

 From the fact that there are a great number of variations in shell 

 character for this sub-family it is necessary to admit several 

 genera so that there may not be so much opportunity for the same 

 types of shell to turn up and thus give false impressions of rela- 

 tionships. It is very striking to note the atavism of the spineless, 

 subovate glochidium of this sub-family in the fact of its homolo- 

 gous recurrence under the Lampsilis type. However, this natural 

 reversion to primitive type in the embryo of the Lampsilinae is 

 only an indication of the wide gap between the two sub-families 

 as well as in the fact of its dilTerences of physiological characters 

 in the adult, such as the discharge of glochidia through the anal 

 opening for the Unioninae and through the branchial for the 

 Lampsilinae. However the homologous differences in the soft 

 parts and hard parts of the two groups are still greater than the 

 analogous. Why more species of this primitive group should 

 occur in the more modern region of this state (i. e., N. Mo., the 

 New Prairies) than in that of the more ancient geologic formation 

 (i. e., S. Mo., — the Ozark Uplift) — this is a problem that the 

 author is trying to solve. The unusual variations within the 

 sub-family especially is another problem that would also be 

 solved. 



