136 



UNIONISE. 



The fresh-water mussels, as the members of this family 

 are popularly called, are very closely allied to the marine 

 mussels, and differ chiefly in the structure of the foot of 

 the animal which, in the tribe before us, is greatly deve- 

 loped in dimensions, and is not provided with a byssal 

 groove. Mr. Anthony, an American conchologist, how- 

 ever, asserts that under peculiar circumstances certain 

 North American species do spin a byssus both in the young 

 and adult states.* As in several of the genera of Mytilida, 

 the mantle has its margins, which cannot be reflexed, 

 freely open almost throughout, and the siphons are only 

 indicated by a difference in the structure of the posterior 

 borders, the shells vary greatly in form and aspect. 

 Those of our few native species are more or less oblong, 

 and depressed, but so great is the variation among foreign 

 species, that an American naturalist observes, there seems 

 to be a representation of the forms of all the genera of 

 marine bivalves in this tribe. All the Unionidez are equi- 

 valve. They are covered with an epidermis which is often 

 remarkable for brilliancy of colour, a feature also of the 

 pearly surfaces of the insides of their valves. 



The variation of the hinge is very great in this tribe, 

 and its value as a source of character has been a subject of 

 much discussion among conchologists. Some, as Mr. G. 

 * Letter to J. E. Gray, in Annals Nat. Hist. vol. vi. 



