LIMNEA. 



acutely turrited. In most of the species the aperture is 

 large, longitudinal, and entire, that is, not emarginate, 

 nor does the lower part of the last volution ever form a 

 considerable sinus in the upper and left side of it : it is 

 generally rather narrower at its upper than at its lower 

 extremity. The outer lip is sharp-edged ; it turns to the 

 left at its lower part, and then ascends and spreads more 

 or less over the ventricose lower part of the last volution, 

 covering the columella, and forming with it an oblique 

 obtuse fold. The external part of the shell is smooth, 

 sometimes polished. 



V In all the above characters, we find the Genus jP/ty^a, 

 of Draparnaud, agrees perfectly; we know of only one 

 mark of discrimination in the shells themselves, it is that 

 the Physae are heterostrophe shells ; but since we know many 

 instances of heterostrophe monstrosities in various genera, 

 this surely cannot be considered a good generic distinction. 

 Accustomed as we were to receive and adopt many Genera 

 without sufficient examination, we could not have anti- 

 cipated this difficulty, for we thus find ourselves obliged 

 either to unite two genera which have appeared distinct to 

 Lamarck and Draparnaud, and which have been adopted 

 by some succeeding writers, or, contrary to our wishes, 

 and as we think to the interests of conchological science, 

 we must not only continue to separate the Phi/sce from the 

 Limnece ; but we must also adopt Fleming's Aplexa^ and 

 Leach's Myxas^ each of which would, as far as we yet 

 know, only contain one species. These are all fresh- water 

 shells; and the only describable difference in the shells, 

 except mere specific difi'erences, consists in the Aplexa and 

 Physa being heterostrophe shells, while the Limnea and 

 Myxas are dextral. Greater differences are found in the 

 animals, chiefly in their tentacula, and in their mantles ; 

 the Myxas of Leach, and the Physa of Draparnaud, hav- 

 ing the power of extending the edges of their mantle over 

 a large portion of the external part of their shell, which 

 the Limnea of Lamarck, and the Aplexa of Fleming have^ 

 not, while the tentacula of all, but Physa, are com- 

 pressed and triangular, and even in Physa they are com- 

 pressed according to Lamarck, though filiform: in all of 

 them the eyes are found at the internal base of the tenta- 

 cula, supported on very short tubercular pedicles. 



The facts we have mentioned above, induce us, there- 

 fore, to unite the whole of these shells under the generic 



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