MOLLUSCA OF INDTA. il 
putra, where they are elevated to 10,000 feet in the Patkai and 
Naga Hills. Inthe Garo Hills this dimishes to 3000 feet, but they 
are there in force with a thickness of some 5000-6000 feet: vide 
‘Journal Asiatic Society of Bengal,’ vol. xxxviil. pt. 2, no. 1, 
1869, with a Geological Map of a portion of the Khasi Hills 
near longitude 91° E. 
Connected with this range of the genus, two facts stand out :— 
(1) The extreme age of the great valleys; (2) the great difference 
between the Molluscan fauna of Sikhim and that of the Dafla Hills, 
still more when it is compared with that of the Arbor country. 
There are very few species common to both. Few Sikhim species 
are found in either: all is new, even new genera come in. The 
reason for this is no doubt due to the physical features of the 
great valleys: some, such as the Monass and Subansiri, are very 
broad; they go back far into the Range; their sources glacial, 
they are separated one from the other by lofty snow-covered 
longitudinal ranges, which continue high to the plains. They are 
thus completely isolated one from the other, allowing evolution to 
go on independently within them and form “ specific centres.” 
The rich flora and fauna of Sikhim is in direct relationship to 
its position at the head of the Bay of Bengal, and for ages has 
received accessions from that, the Southern side; so with species 
of Glessula, when those at present living between the Teesta and 
the Monass are compared with those of the Khasi and Garo Hills, 
100 miles to the south, how small and yet how defined is the 
difference. 
Himalaya area : Represented in the Khasi-Garo area: 
longispira, tenuispird. 
hastula. subhastula. 
baculina. sub-baculina. 
Between these two areas there is an indication of a once more 
continuous land-surface higher that at present. All this delta area 
has gone through considerable depression with denudation, This 
is So well exemplified by the isolated, weathered masses of intrusive 
granite rising abruptly out of the alluvium by which they are 
surrounded at Chanda Dinga, opposite Gwalpara and Doobri. My 
Survey work took me to the top of several such hills. Granite 
intrusion is frequently seen ; it is to be noted at Tura and Riwuk 
on the Assam Range, and similar intrusions occur further east and 
north intimately connected with the forces of upheaval. Those near 
Gwalpara, on the north side of the Brahmaputra, no doubt originally 
passed up and through stratified rocks long since denuded (perhaps 
of Cretaceous age) which cover so large an area in the Garo Hills, 
where they have also suffered great denudation. 
Numbers, followed by the letters B.M., refer to specimens 
catalogued in three collections presented to the British Museum, 
viz. those of 1. W. T. Blanford and H. F. Blanford combined, 
2. Colonel H. Beddome, 3. Godwin-Austen; they cannot fail 
to assist those who may study this group or have to name specimens 
from India. 
