AKT. 11 ANATOMY OF FEESH-WATER MUSSELS REARDON 9 



smooth pads. The inner lamellae of the inner gills are free from the 

 visceral mass. Hinge teeth are absent and the muscle scars faint. 

 The shell is rather thin, evenly rounded in front and somewhat 

 biangulate behind and is very slightly winged in the postero-dorsal 

 region. The beaks are rather full, the sculpture consisting of a 

 moderate number of concentric ridges which are usually doubly 

 looped. The surface is usually covered with irregular growth lines. 

 The exterior is generally shining, greenish-yellow, yellow-green or 

 olive, usually banded with darker color and often faintly rayed. The 

 interior is bluish-white and not shining. There are no small, dorsal 

 muscles in Anodonta. 



MANTLE 



Plate 3, fig. 3 



The mantle edge is strongly pigmented at the siphons. The 

 connection of the mantle edge runs nearly the length of the posterior 

 adductor muscle, restricting the dorsal mantle aperture to a very 

 short slit and causing it to lie behind the anterior adductor muscle. 

 The tentacles of the inhalent siphon are large wdth some of them 

 forking. The exhalent siphon has no tentacles. 



GILLS AND REPRODUCTIVE SYSTEM 



Plate 3, figs. 6, 7, and 9 



The gills are semicircular with the inner lamellae of the inner gills 

 free from the visceral mass. The entire outer gill serves as a marsu- 

 pium. During the breeding season the water tubes of the marsupium 

 are divided longitudinally into three tubes with the two outer tubes 

 facing the inner and outer lamina. It is only the central tube which 

 contains embryos. This threefold division of the water tubes is 

 present only during the breeding season. 



The glochidia oi A. cataracta are of the hooked type. 



BEAK SCULPTURE 



Plate 3, fig. 5 



Elliptio productus Conrad and Elliptio complanatus (Solander) 

 Dilhvyn. 



In this genus three species of the fisherianus group, Elliptio 

 productus, E. Hslierianus, and E. lanceolatus have been thought to 

 occur in the District of Columbia. Close examination of both shell 

 and morphology shows that there is no constant distinction, and it is 

 therefore thought that the only representative of this, the JisJierianus 

 group, in the District of Columbia is E. productus Conrad. 



The group of Jisherianus is distinctive in the manner of gill attach- 

 ment, for the outer lamina of the inner gill, instead of having the usual 

 connection to the visceral mass, is free. 



