314 CYPRINID,!:. 



ground : doubtful specimens occur where they might be 

 looked for — viz. on the confines of two localities, each 

 inhabited by its own form. The above remarks equally 

 apply to the A. incrassata of Brocchi, except in its being 

 a southern form. Philippi at first considered it the same 

 as our species, but he afterwards changed his opinion 

 on the authority of M. Koch. Deshayes, in his edition 

 of Lamarck, represents A. incrassata and the Tellina 

 fusca of Poli as two distinct species ; but Philippi demurs 

 to this, on the ground that the extent of sulcation and 

 the comparative convexity of the shell are very uncer- 

 tain characters. Such a conflict of opinion is easily 

 settled by pronouncing the three species to be distinct ; 

 but I believe the sounder judgment will be that all are 

 identical. It is impossible to ascertain, or even to 

 conjecture with any reasonable probability, how much 

 change prolific and hardy species, like the present, may 

 have undergone during the lapse of countless genera- 

 tions ; 



" For formes are variable, and decay 

 By course of kind and by occasion." 



The A. elliptica of the north, A. sulcata of more tem- 

 perate seas, and A. incrassata of the south are evidently 

 so closely allied to one another that it is not unphilo- 

 sophical to suppose that they originated from the same 

 common stock. The slight differences which they pre- 

 sent among themselves may have been caused by local 

 or accidental conditions. I need not apologize for par- 

 ticularizing so many varieties, as all naturalists are 

 agreed as to the utility of this mode of discrimination. 

 The time has gone by when varieties were not regarded. 

 At present the course of scientific inquiry tends the 

 other way ; and varieties must be named, or have some 

 equivalent symbol of distinction. 



