ANATINA. 



presence of which, in a free bivalve, constitutes the essen- 

 tial character of the genus) is very small and of a curved 

 semilunar shape; in others it is proportionately large, 

 though of the same shape; sometimes again it is flat and 

 placed without the intervention of any cardinal processes 

 against the internal ligament. Notwithstanding this 

 remarkable diversity, we cannot consent to separate any 

 of these from the rest, because we are convinced, that if 

 any separation were allowed, it would be necessary to 

 raise nearly every known species to the rank of a genus. 

 Wherefore, as far as regards those bivalves which have 

 an internal testaceous appendage, we are satisfied with 

 separating frona Anatina only the Myochama and the Clei- 

 dothcerus, both of which differ from it in being fixed. 



The Anatince may therefore be described as free, 

 transverse, inequilateral bivalves, generally with unequal 

 valves; sometimes gaping, sometimes nearly closed at 

 both ends, generally having a small spoon-shaped inter- 

 nal process, to which the ligament is attached, in each 

 valve ; in connection with which there is a small, variously 

 shaped, testaceous,internal appendage also attached to the 

 ligament, and apparently serving to strengthen the con- 

 nection between the two valves. 



Fossil in the tertiary beds only. All the species are 

 marine. 



