UNIONID&. II 



readily be distinguished from it and the other members 

 of the genus by its oblong shape, by its beaks being 

 placed away from the centre, by the straight outline 

 of the lower margin, and finally by the absence of 

 striae near the beaks, or of any appearance of notching 

 round the margin of the siphonal orifice. 



FAMILY II.-UNIONIDJE. 



Body oblong, compressed ; mantle open, except behind, where 

 it forms two orifices. Through the upper and smaller of these 

 orifices, the edges of which are simple (not fringed), the ex- 

 cretions of the animal are voided, and it is separated from the 

 lower and larger one by a plait or fold of the mantle ; the lower 

 and larger orifice, by means of which the animal breathes, is not 

 simple like the other, but is furnished at its margin with several 

 rows of threadlike filaments, which serve as tentacles or feelers ; 

 mouth as in the first Family, consisting of a slit with two small 

 triangular lips, and placed between the anterior (front] adductor 

 muscle and the base of the foot ; foot large, broad, tongue- 

 shaped. 



Shell " equivalve, oblong, inequilateral, compressed ; epider- 

 mis thick ; beaks (which form the nucleus, or young shell) plaited 

 or wrinkled ; ligament external, strong, and always conspicuous ; 

 hinge furnished with, lateral teeth only ; those on the anterior 

 side being sometimes so much developed as to resemble cardinal 

 teeth." B. C., vol. i. p. 28. 



The Unionidce, popularly known as "freshwater 

 mussels,"' unite the two sexes in the same individual, 

 in other words, each animal is both male and female. 

 Like most of the molluscs, some of them are ovipa- 

 rous, while others, as is the case with the Sphceriidce, 

 are ovoviviparous. They live in rivers, lakes, and 

 ponds, and feed upon small aquatic animals, espe- 

 cially the minute freshwater Crustacea, which in some 



