214 BRITISH MOLLUSKS. 



with brown about the edge, particularly m the parts surrounding 

 the branchial orifice. 



The lakes, canals, ponds, and gently flowing rivers throughout 

 Europe, are all tenanted by the Anodonta in some form or other. 

 It feeds on decomposed animal and vegetable matter, and the shell 

 varies according to the quantity and character of the food, the still- 

 ness or disturbance of the water, its chemical composition, depth or 

 shallowness, and other causes, From forty to fifty species have 

 been made of these different phases of the European Anodonta, but 

 our best authorities on the subject, Messrs. Forbes and Hanley, and 

 Dr. Gray, are of opinion that they are all referable to one. Eleven 

 more species have been recorded as inhabitants of the Eastern 

 Hemisphere, eight Asiatic, chiefly Philippine, two African, and one 

 Australian. The Western species Mr. Lea enumerates as eighty- 

 two, of which thirty-two are South American, and fifty North 

 American ; A. fluviatilis, inhabiting the western and central parts 

 of Massachusetts, being almost identical with our own species. 



Dr. Gray's testimony in favour of there being only a single species 

 of Anodonta in Britain is expressed in the following able manner : — 

 " The Anodons feed on decomposed animal and vegetable sub- 

 stances ; and the size and solidity of the shell depends on the 

 abundance of the food and the state of quietness or motion and of 

 calcareous matter in the water in which they happen to reside. 

 Some authors have believed them to be unisexual ; but their ana- 

 tomy proves that they are hermaphrodite and sufficient for them- 

 selves. Poiret supposed that of the two species he observed near 

 Paris, one was viviparous and the other oviparous ; but they all 

 deposit eggs, which are developed in their exterior pair of gills. 

 They have been divided into numerous species ; but in ponds where 

 there is plenty of food (and a dead dog or cat or fish affords abun- 

 dance of such material), and where the water is nearly stagnant 

 and seldom disturbed, they become of a large size, with ventricose 

 thin shells, while in more rapid rivers with pure clear water, with 

 very little decomposed animal or vegetable matter held in suspen- 

 sion, they are small, with compressed thick shells ; and all inter- 

 mediate forms and sizes are to be observed. After collecting many 

 hundred specimens from various localities, I am convinced that 

 there is only a single species found in this country." 



Our British Anodonta is : — 



1. cygnea. Shell a pair of thinly ventricose toothless valves, 

 slightly auriculated on the posterior side. 



