DISTRIBUTION OF GENERA AND SPECIES. Xg5 
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 
lAmaeidce, of which Tebetmophorus is the only unequiv- 
ocally native genus, while Avion and Vaginulus are 
each represented by a single introduced species, and 
Limax has two introduced species and one somewhat 
doubtful native. The whole appear to have been for- 
merly represented by Tebennophovus, 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 Clausilia, 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 Cylindvclla, which is common in the 
Antilles. Vitvina contains only a single species, iden- 
tical with the most common European species, and by 
many thought to be introduced. Of the other genera 
common to both lists, Succinea is more numerously rep- 
resented here than in Europe; Pupa, Vevtigo, 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 
vol. i. 43 
