PLATE XLIX. 



situated on the right side, and sometimes on the left, the wreaths of 

 these two shells will be inverse to each other, and these are the 

 " dextra " and sinistra " varieties or species of authors. The 

 most frequent of the varieties of Helix Perversa are those which turn 

 in an inverse direction, and the rarest those whose volutions are in 

 accordance with spiral shells in general. This difference in the di- 

 rection of the spire can by no means constitute a specifical distinction 

 when the shells are in all other respects the same. Few species of 

 the testaceous tribes are constantly reverses ; there are some and 

 reverses of those shells are rare as well as contrary shells of those 

 which usually wreath in the ordinary manner ; examples however do 

 occur sometimes, and in sufficient number to assure us that this cir- 

 cumstance alone can be no criterion of a species, and that probably 

 no species can be deemed exempt from such an accidental deviation 

 in its structure. 



With respect to the varieties of this shell they appear reducible 

 to two primitive kinds, the immaculate and the striped or spotted. 

 The immaculate kind is of a yellow colour, varying to a pallid tint ; 

 sometimes the yellow is tinged or clouded with a pale greenish hue, 

 more or less inclining to blueish, while in others it is of a rich and 



* The species Turbo perversus are constantly reversed or contrary 

 shells. Donovan^s Br. Shells, vol. 2, t. 72. A reversed kind of Helix 

 pomatia occurs in Hungary pretty frequently, while in England the spire 

 of the same species turns as usual in other Helices. Br. Shells, \o\. 3, t. 84. 

 Reversed shells of the Helix hortensis, or garden snail, are rare, we have 

 heard of three examples, one of which was valued at ten guineas ; we 

 have also seen Helix arbustorum reversed. Other instances might be pro- 

 duced among the spiral shells of several different genera, but the above may 

 be considered sufficient to prove that the mere circumstance of being 

 reversed is no evidence of specifical difference. 



