PLANORBIS. O 



Planorbis appears to be more nearly related to Helix, 

 than any other genus of Physadas; in the first place, by 

 the form of the shell, and secondly, by the dentate aperture 

 in the subgenera Segmentina and Planorbula. Some au- 

 thors are inclined to look upon this armature as an artificial 

 character, only useful for specific distinction; in other 

 words, characters, the bearings of which are not under- 

 stood, we, in <feur ignorance, call artificial; and arrange- 

 ments which take them into account, are designated by the 

 same term, to distinguish them from natural arrangements, 

 founded more or less upon appreciable relations.* It is 

 certain, that the more that is known, the less is it possible 

 to draw a line of distinction between the two assumed orders 

 of character. 



We might, at first view, suppose size and color to be 

 artificial characters; at least, as far as genera are con- 

 cerned; yet we find Paludina composed of comparatively 

 large individuals, and the allied genus Amnicola, of small 

 ones. Certain uniform colors, lines, or spots, mark genera 

 and families; in Cervus and Antilope the rump is frequently 

 of a lighter tint than the general color; and in Cicindela, 

 the pattern is remarkably uniform through a very large 

 number of species. Under Melania, some remarks will 

 be found upon geographical position, as indicating the 

 genus, in certain cases where species present anomalous or 

 osculatins; generic characters. 



* Methodus artificialis exceptionum quidem immunis esse 

 potest, quia ex unitate principii divisionis derivatur; sed 

 systema naturale exceptionibus necessario abundat, ad cha- 

 racteres autem, non ad affinitatem, pertinentibus. — Jlgardh, 

 Classes Plantanim. 



