INTRODUCTION. XVil 
It is highly probable that further search would yield many 
additional species. 
The geological systems above alluded to comprise many 
formations and sub-divisions. Though very complex—ranging 
from the Old Red Sandstone to the Chalk-—yet, from an eco- 
logical standpoint, they may be classed under the three headings 
of sandstones, limestones, and deep marls and clays, as pointed 
out by Dr. C. E. Moss in his admirable paper on the Geo- 
graphical Distribution of Vegetation in Somerset.! I have 
drawn largely upon his paper for my botanical notes in the 
subjoined brief survey of the inland mollusca of the county, 
and have much pleasure in here acknowledging my indebted- 
ness, and expressing my appreciation of its value. 
Dr. Moss observes that ‘* Hast Somerset has a slightly higher 
summer temperature and a slightly lower winter temperature 
than West Somerset, and also from the geological and botanical 
standpoints, Hast Somerset has more in common with eastern 
England than with south-western England.” An examination 
of the molluscan fauna gives zoological support to this con- 
clusion, e.g. we do not find Jaminia anglica in the eastern part 
of the county ; other examples might be quoted. 
Considered as a whole, the mollusca of Somerset belong to 
the Lusitanian group, the term “ Lusitanian” being under- 
stood to include the extreme south-west of Europe (and north- 
west Africa), and not limited to Lusitania of Roman times, 
which included only a large area of Portugal. A “ Lusitanian” 
mollusc is one which has migrated from South-W estern Europe 
to Central, Southern, or Northern Europe, either in _pre- 
glacial times or later. Forbes considered that the Lusitanian 
elements are the oldest of the components of our existing 
fauna and flora, and date from Miocene times. Molluscs of 
this group occur chiefly, as far as the United Kingdom is 
concerned, in the mountainous districts of the south-west and 
1. Royal Geographical Society, 1907. 
