DISTRIBUTION OF GENERA AND SPECIES. IQ^ 



ities can be supposed to have the advantage is, the more 

 equable and gradually changing temperature which they 

 enjoy; the changes of temperature in many parts of 

 this country being frequent, rapid, and excessive. 



We notice, also, the remarkable deficiency of the 

 lAmaeidm, of Tvhich Tehennophorus is the only unequiv- 

 ocally native genus, while Arion and Vaginulus are 

 each represented by a single introduced species, and 

 lAmax has two introduced species and one somewhat 

 doubtful native. The whole appear to have been for- 

 merly represented by TehenyiopJiorus, which is believed 

 to be peculiar to North America, and differs essentially 

 from its allied genera by its external form and internal 

 structure. It may be remarked, however, that as the 

 animals of this family are nocturnal in their habits, and 

 their places of resort are but little open to common ob- 

 servation, it is almost certain that other species will be 

 discovered. The genus Olaiisilia, which, in Europe, 

 comprises numerous species, is without a representative, 

 and, so far as we know, does not occur either upon the 

 continent of America, or the adjacent islands. Its place 

 is supplied by Cylindrellay which is common in the 

 Antilles. Vitrina contams only a smgle species, iden- 

 tical with the most common European species, and by 

 many thought to be introduced. Of the other genera 

 common to both lists, Sucdnea is more numerously rep- 

 resented here than in Europe ; Pupa, Vertigo, and Buli- 

 mus, much less so, although our proximity to the regions 

 where Bulimus prevails would induce us to expect to 

 find it more numerous ; and Helix, making allowance for 



