40 UNIONIDJE. 



The habits of the Anodontce are the same as those of 

 the Uniones ; but they differ, according to Moquin-Tan- 

 don, in being ovoviviparous. 



It is by no means an easy task to distinguish some of 

 the species of Unio ; but the difficulty is much greater 

 in attempting to separate the various forms of Anodonta. 

 Even the great Danish naturalist, Miiller, entertained 

 grave doubts, nearly a century ago, whether there ex- 

 isted more than one Scandinavian species ; although his 

 hesitation was not participated in by Nilsson and subse- 

 quent writers on the Mollusca of that country. In other 

 parts of the Continent, the long array of specific names, 

 which have been recorded by H. Drouet, shows that the 

 tendency of modern conchologists has been vastly to 

 increase the number of European species. In this 

 country, Montagu, with all his powers of discrimination, 

 evidently entertained considerable doubt as to the spe- 

 cific difference between A. cygnea and A. anatina ; and 

 Turton, more than thirty years ago, expressed his opinion 

 "that all our supposed species of this genus may be 

 justly resolved into one, varying in their outline, con- 

 sistence, and colour, from age and local circumstances." 

 This view has been adopted by Dr. Gray and the authors 

 of the ' British Mollusca.' However, as long as any di- 

 stinction of species is recognized, we must endeavour to 

 deduce from the observation of natural phenomena any 

 facts which may facilitate such investigation. One of 

 these facts seems to consist in ascertaining whether any 

 different forms inhabit together the same spot and under 

 exactly similar conditions, without any appearance of an 

 intermediate link or gradation. Montagu has recorded 

 such a fact with respect to his Mytilus avonensis and 

 M. anatinus ; Drouet has given other instances of the 

 collocation of several species of Anodonta in French 



