Ortmann; Families vnd Genera oi Najades. 



285 



anterior end. Posterior margins of palpi connected for no1 quite 

 one-half oi their length. 



an 



,br 



Fir,. 12. Arcidens confragosus (Say). Male from Bayou Pierre, De Soto Parish, 

 La. (Cam. Mus., No. 6i, 4.701.) 



Gills anodontine in structure. Simpson (1900/?, p. 661) describes 

 the marsupium in peculiar terms ("of a peculiar, granular texture"). 

 Although I have not seen gravid 

 females, the sterile females I 

 possess offer nothing unusual or 

 different from other Anodontincc. 

 Only the outer gills are marsu- 

 pial, and their septa are much 

 crowded, forming very narrow 

 water-tubes, while in the inner 

 gill the septa are much more 

 distant. The septa of the outer 

 gills are typically anodontine, 

 and an indication of secondary 

 water-tubes is present in the 

 sterile female (see pi. XVIII, 



mp 



Fig. 12a. Left gills of a sterile fe- 



fig. 6). Besides, at the edge of malc - from Pearl Rivcr ' J ackson - Hinds 

 , . . ... , . Co., Miss. (Cam. M us., Xo. 61, 4,928.) 



the marsupial gill, there is a 



thick mass of tissue, which indicates, that in the gravid female the 



edge is capable of distending. 



