﻿PTYCHOBRANCHUS 333 



branchial opening large, with very minute papillae or crenula- 

 tions, sometimes smooth ; anal opening crenulate or smooth. 



Type, Unio phaseolus Hildreth. 



A small group of species confined to the Mississippi and 

 Alabama River drainage systems, excepting P. phaseolm, 

 which extends into the St. Lawrence area. The shells are not 

 remarkable, being rather solid and varying from triangular to 

 long elliptical, having generally broken or wavy rays and club- 

 shaped laterals. The animal differs but little in obvious char- 

 acters from that of Unio, but at the time when the female is 

 gravid a most marvelous change takes place in the outer gills. 

 The plain surface becomes gradually wavy and is then filled 

 with embryos occupying distinctly marked ovisacs, each hav- 

 ing a sort of bulb at its base, and when in this condition the 

 entire outer gills hang in a series of remarkaljle and beautiful 

 folds like those of a rich, heavy curtain. As soon as the em- 

 bryos are discharged the lower borders of the gills contract 

 until they are merely wavy and they finally become perfectly 

 plain again. 



Kkv to spr:ciKS O'F PTYCH0I!RANCHUS. 



Shell long-elliptical or long-triangular. 

 Surface smooth. 



Tawny, with or without broken rays. P. phaseolus. 



Greenish, with delicate, wavy rays. P. clintonense. 



Surface plicate posteriorly. P. subtentnm. 



Shell short or of moderate length, triangular. 

 Tawny, with or without broken rays. 



P. foremanianum; greeni. 

 Dirty green, feebly rayed, inflated. P. trinacriuyn. 



Ptychodranchus phaseolus (Hildreth). 



Shell long elliptical, long ovate or long subtriangular, in- 

 equilateral, solid, subcompressed to subinflated; beaks low, 

 sometimes slightly inflated, their sculpture very faint corru- 

 gations ; posterior ridge generally moderately developed ; sur- 

 face with irreg-ular, strong growth lines ; epidermis tawny, 

 having a few broken, green rays, sometimes rayless ; left valve 



