W. F. R. Weldon 
123 
At the risk of wearying the reader by repeating notorious facts, I wish to 
illustrate the relation between geographical distribution and specific characters 
by reference to a country which I know better than I know Holstein : and I 
therefore publish, in Table V., the mean lengths of the peripheral spiral radii 
in the upper whorls of a small series of adult C. laminata from a beech-wood 
in the neighbourhood of Monk's Risborough, in Buckinghamshire. 
TABLE V. 
Peripheral Radii of 32 Adult Glaiosilia laminata from Risborough. 
Angular Distance 
from plane of 
Standard Columel- 
lar Kadius in 
right angles 
Mean Peripheral 
Kadius 
-19 
0-8967 
-17 
11318 
-15 
1-4036 
-13 
1-6888 
-11 
2-0240 
- 9 
2-3686 
- 7 
2-7685 
- 5 
3-1741 
- 3 
3-6576 
- 1 
4-1395 
The measurements of this race are not yet completed, and it is not worth 
while to discuss the probable errors of the small number of measures available ; 
but a comparison between the results so far obtained and those recorded in 
Table III. (see also Fig. 3, p. 114) gives the impression that the races at Grems- 
miihlen and Risborough are sensibly identical in the mean character of their 
peripheral spiral. 
The environmental elements common to the beech-wood near Monk's Ris- 
borough, where these shells were found, and the wood at Gremsmlihlen, are 
those directly connected with the presence of beech-trees, and apparently few 
others. The wood (or copse) at Risborough is 800 feet above the sea level, on 
the steep side of a chalk down : it is of very small extent, and the beech-trees 
are tiny and young compared with the magnificent old trees at Gremsmuhlen. 
The dryness of the ground is shown by the total absence of Siiccinea ; while a 
difference of conditions important to terrestrial molluscs is shown by the abund- 
ance at Risborough of Gyclostoma elegans, a species which cannot live in the 
sandy plain of North Germany. The species of Helix, mentioned as common in 
Gremsmuhlen, also occur in the wood at Risborough, but the prevalent colour 
varieties of H. nemoralis are different, and thei-e are well-marked differences 
between the races of H. arbicstorum in the two localities. In addition to these 
species, H. aspersa, which does not occur east of the Rhine, and H. cantiana, 
