﻿STROPHITUS 345' 



The presence of vestigial pseudocardinals distinguishes this 

 group conchologically from Anodonta. But at the time that 

 the embryos enter the outer gills a most remarkable trans- 

 formation of these organs takes place. Instead of these em- 

 bryos being contained in vertical or oblique ovisacs as they are 

 in all the other known Naiades, they occupy short ovisacs that 

 run horizontally across the gills ; break through the outer walls 

 of the gills and are discharged entire with their contents into 

 the water. As soon as this has taken place the gills again 

 assume the ordinary appearance of those organs in the Union- 

 idse. 



Key to species of Strophitus. 

 Shell smooth. 



Shell rayless or only feebly rayed. 



Rather solid, blackish or brownish. S. edcntulus. 



Rather thin, smoky-brown. S. iiiidiilatus. 



Yellowish, tawny or greenish-yellow. 



S. counosaugaensis, gcsncri, alahamensis. 

 Shining, tawny or olive with dark bands. S. spillmanii. 

 Trapezoid, narrow in front, dark, dull colored. 



S. tombigbecusis. 

 Shell rather brightly rayed. 



Oblong oval. S. edcntulus pai'onius. 



Widely biangulate behind. 6'. iindiilatus quadriplicatus. 

 Long rhomboid, truncate at anterior base. S. radiatus. 

 Shell plicate on dorsal slope. 



Folds strong. S. ivriglitiaunus. 



Folds delicate. S. subzr.vus. 



Strophitus edentui^us (Say). 



Shell long elliptical to long rhomboid, subinflated, subsolid, 

 with moderately full, high beaks, whose sculpture consists 

 of a few very coarse ridges, which run nearly parallel with 

 the growth lines and turn up slightly behind ; posterior ridge 

 variable, generally angular ; surface usually with strong, irreg- 

 ular, ridge-like growth lines ; epidermis black or brownish, 

 rayless or rayed, scarcely shining; hinge line generally rather 



