VOL. I. | Variation in Shells. : 335 
| Helix Californiensts Lea. 
Largest specimen : greatest diameter, 25 mm.; altitude, 28 mm. 
Smallest specimen: greatest diameter, 16 mm.; altitude, 19 mm. 
Color rigid, band frequently absent. 
I might continue this list almost indefinitely by showing that 
colonies of widely-distributed species existing in the very metropo- 
lis often exhibit the extremes of variation known to it, but enough 
has already been given to show that variation has no geographical 
bounds, and that the causes of this phenomenon of nature are active 
in limited as well as in extended areas. 
I will now briefly allude to another point closely connected with 
the study of variation. The origin of some of our largest groups of 
land snails seems to be a mystery to some of the writers upon our 
conchology, and more especially the group found in California, which 
they are pleased to call “Arionta.” The history and origin of that 
group of shells, as I have traced it from the north, is as plain to me 
as a well-worn road at midday. 
The northern portion of the Rocky Mountains, so far as I have 
been able to trace, seems to be the region that has sent out colonies 
of shells, which have spread over nearly the entire country to the 
south, east and west; and there exist to-day, in the mountains of 
Idaho and Montana, active and vigorous colonies of shells that seem 
to me to be the progenitors of our present shell fauna, or at least of 
the larger part of it. 
Among these Helix Townsendiana var. ptycophorus Brown, is the 
form from which have evolved the two groups we know as Mesodon 
on the eastern slope, and the group we call “Arionta,’’ on the 
western slope. This shell is quite variable in size and coloring in 
its metropolis, and in its travels southward it assumes a slightly 
modified form, color and sculpturing, as new conditions require. 
In the moist and heavy timbered regions of Oregon it becomes 
larger and more strongly wrinkled in sculpture, and is known as 
Helix Townsendiana Lea. When this reaches its southern limit 
near Humboldt Bay, California, it has again become somewhat 
modified in form, and is then known as /7. arrosa Gld. _Continu- 
ing its march southward it eventually reaches Sonoma and Marin 
counties, and here, under the influence of climatic and geological 
changes, it branches out and sends off colonies in several directions, 
_ which we know under the following names: 
