MOLLUSC A. 
115 
Current, as well as to a passage which formerly existed across Africa 
ln the line of the Sahara — a very wide tract, which certainly was 
submerged during the quaternary period. I must admit, however, 
that our information as to the marine Mollusca of the South- 
Atlantic region, including St. Helena, is very scanty and unsatisfactory. 
1 he only dredging that has ever, to my knowledge, been attempted 
off St. Helena was made by Dr. Wallich in 1857, on his return home 
from India ; and this was at a depth of from twenty to thirty fathoms. 
It produced a few small shells, which Dr. Wallich kindly gave me. 
Many of these appear to be undescribed species. The promised 
circumnavigation expedition, under the auspices of the Boyal 
ociety, will doubtless enable us to learn something of the South- 
Atlantic fauna. 
Mi . Edgar Smith will describe such of the species, and those 
dredged by Dr. Wallich, as are new to science.” 
Ol the land-shells eleven species are now found in a living state, the 
^est are all in a more or less subfossil condition, embedded in the super- 
ficial soils of the upper parts of the Island, varying at heights of 1200 
0 700 feet above the sea, and entirely confined to the north-eastern 
quarter of the Island. Mr. Darwin, who visited the Island thirty-eight 
years ago, attributes their extinction possibly to the loss of food 
u shelter they experienced by the destruction of the native woods 
lck occurred during the early part of last century, when the old 
•, ees an d were not replaced by young ones, these being destroyed 
q- frm goats and hogs, which had run wild in numbers from the year 
The state of the shells and the positions in which they 
ar e now found on the barren parts of the Island seem to indi- 
cate that such was the case with the last surviving members of the 
amily, but I am inclined to date the commencement of their decline 
° a more ancient period — viz., that time when those parts where the 
1 ls are now found were swamps clothed with vegetation and such 
j < j mouts as were essential to the existence of Bidimi, but which, as the 
and became smaller through the encroachments of the sea, lost 
J-ear moisture by drainage and consequently with it their vegetation 
c suitability to sustain snail life. 
These beds of extinct land-shells, which occur chiefly on Flagstaff 
* Journal of Researches. By Charles Darwin, M.A., F.E.S., p. 582. 
