1 8 NAIADES OF MiSSOURT 



Tennessee basin may furnish some interesting data for the recon- 

 struction of ancient geographical features for the central Missis- 

 sippi Valley. 



Family II. Unionidae vSwainson (restricted). 



"Diaphragm complete, formed only by the gills; the outer 

 lamina of the outer gills connected with the mantle at its posterior 

 end. Anterior end of inner gills separated from palpi by a more 

 or less wide gap. Margins of the mantle held together by the gill- 

 diaphragm, but not united, thus separating the anal from the 

 branchial opening, and the anal is generally closed above by the 

 union of the margins of the mantle, (it rarely remains open), 

 and when closed, it always leaves a supra-anal opening (which 

 is very rarely obliterated). Gills always with water tubes formed 

 by interlamellar connections developed as continuous septa, 

 running parallel to the gill-filaments. Marsupium formed by all 

 four gills, or by the outer gills alone, or by parts of the outer gills. 

 Glochidia of various shapes, suboval, or subtriangular, or celt- 

 shaped, with or without hooks on the ventral margin." — (Ortmann 

 1912b.) 



Simpson's terse diagnosis of this family is: — "Hinge with schizo- 

 dont teeth; embryo a glochidium." 



The family, Unionidae, naturally falls, into three divisions 

 on the basis of physiological and morphological characters; how- 

 ever, this family may fall into two sub-divisions on the sole basis 

 of reproductive functions. The Unioninae and Anodontinae 

 would form the first and the Lampsilinae the second group from 

 the fact that the discharge of the glochidia takes place in the 

 former through the primitive and natural way of passage from the 

 ovisacs through natural openings into the suprabranchial canals 

 and then on out through the anal opening and in the latter the dis- 

 charge is effected in a more direct and seemingly unnatural manner; 

 that is, in the passage from openings forced through the ventral 

 edges of the ovisacs, and thence out through the branchial open- 

 ing. Yet the two sub-families, Unioninae and Anadontinae, have 

 morphological differences in marsupial characters and in struct- 

 ures of the glochidial masses that are correlated with physiological 

 differentiation in breeding habits. On the same grounds, Lamp- 

 silinae is set aside as well a defined group; however the latter, 

 although the modern group, is related to the primitive one Unioni- 



