UNIONID^. 29 



only ; those on the anterior side being sometimes so much 

 developed as to resemble cardinal teeth. 



Some of these mollusks^ which are often called " fresh- 

 water Mussels/^ are ovoviviparous^ like those of the last 

 family, and retain their young within the folds of the 

 mantle for some time before they are finally excluded ; 

 while others are oviparous, like the majority of mollusks. 

 It was for a long time supposed that they were of sepa- 

 rate sexes, and Von Siebold distinguished Atiodonfa 

 cygiiea as the male, and A. Cellensis as the female, of the 

 same species ; but Moquin-Tandon seems to have now 

 proved satisfactorily that both sexes are common to each 

 individual or that they are all monoecious. They in- 

 habit rivers and other large pieces of water. Their 

 habits are tolerably active in the spring, or when in 

 search of a suitable feeding-place ; and by means of their 

 large fleshy foot they are able to traverse considerable 

 distances, leaving a track or furrow in the soft mud. 

 When the water is slowly drained off", or dried up by the 

 heat of summer, as well as in the winter, they bury them- 

 selves in the mud. Their food consists of Entomostraca 

 and other minute animals. According to Mr. Anthony, 

 an American conchologist, who has especially studied 

 the members of this family, some species spin a byssus. 

 It is difficult to separate this family from their marine 

 analogues, the true Mussels, on merely malacological 

 grounds ; but I believe a good conchological distinction 

 (considering the shell to form an integral and important 

 part of the animal) is maintainable in the position of the 

 ligament and beaks. The former is external in the 

 UnionidcB, while it is internal in the MytiUdcs ; and the 

 beaks are nearly terminal in the latter, but in the former 

 they are seldom placed at a less distance than one-fourth 

 from the anterior end. Besides these marks of distinc- 



