18 



These animals live on the surface of the beds 

 of rivers ; free ; and resting in various positions, 

 on the side or vertically, with the aperture below 

 or oblique. They can when necessary bury them- 

 selves in the sand or earth, particularly in winter, 

 and even in summer in the smaller rivers subject 

 to be dried up. They resist drought very well. 

 Their progressive motion is very slow, effected 

 by means of their foot, which as slowly furrows 

 the ground. They are hermaphrodite and very 

 prolific, the eggs are very small and enveloped 

 in a glairy substance, frequently of a yellow co- 

 lour. Many young shells are hatched in the 

 shell of the parent. 



This species has two varieties, viz : 

 Var. 1. Fusca deep brown epidermis, pale 

 nacre. 



Var. 2. Maculata brown spots, nacre almost 

 white. 



SPECIES II. Unio crassa (Elliptic crassa.) 



Syn. Unio crassus. SAY. 



Shell elliptical, slightly ventricose, very thick ; 

 epidermis brown ; nacre white ; lamellar tooth 

 thick, obtuse ; wrinkles distinct. Length f - 

 diam. axis of the breadth. 



This species is figured by Mr. SAY under this 



