334 Mr A. H. Cooke, On Land and Fresh- Water Mollusca. [Feb. 25, 
so the preceding results may be of interest to theorists on the 
constitution of the earth. 
The paper concludes with a graphical representation of some 
of the more important results. 
February 25, 1889. 
Mr J. W. Ciark, PRESIDENT, IN THE CHAIR. 
The following Communications were made: 
(1) On the relationships and geographical distribution of the 
Land and Fresh-Water Mollusca of the Palaearctic and Nearctic 
Regions. By A. H. Cooke, M.A., King’s College. 
THE author stated that the boundaries of the regions con- 
cerned were, from the Molluscan point of view, identical with 
Wallace’s regions, the Palaearctic being well defined, the Nearctic 
scarcely defined at all towards the south. Hence the intrusion of 
at least ten tropical genera into the southern United States, as 
compared with only one tropical genus in Southern Europe. But 
for the accident of the exceedingly small amount of land within 
the tropics in the New World, this proportion would have been 
much Jarger. 
With regard to the Land Mollusca, no subdivision of the 
Palaearctic Region was possible: Great Britain and Amurland 
showed a very large percentage of common species. — 
On the other hand, the genera of the Nearctic Region fell 
into three main groups, (1) genera common to the whole region, 
(2) genera peculiar to the Pacific coast, (8) genera peculiar to the 
K. States. The high table-land between the Sierra Nevada and 
the Rocky Mountains, desert and waterless, interposed a barrier 
between the W. and E. genera of the same nature as the line of 
deserts which defined the Palaearctic region. 
Group (1) was composed of very minute species (which always 
extend over large areas, as compared with species of larger size); 
83 per cent. of these were common to N. Hurope, thus affording 
strong support to the view which established a ‘Holarctic’ region. 
Group (2) or genera peculiar to states W. of the Sierra Nevada 
Showed more affinity to Europe (in the proportion of 45 to 37 per 
cent.) than did group (3), and also showed relations to genera now 
occurring in China. Hence it was suggested that the possible 
closing of Behring’s Straits, and the existence of continuous land 
north of the chain formed by the Kurile and the Aleutian Islands, 
