XX1V INTRODUCTION. 
the general resemblance of the land-shells of the Mexican State of Vera Cruz to 
those of Northern Guatemala forbid the former, the strictly South-American forms 
in Costa Rica the latter. The longer tracts of the Nicaraguan depression, or even 
that of the Golfo Dulce (as regards Eucalodium), will serve the purpose perhaps a 
little better, if we want a separating-line between the South-American and the Mexico- 
Guatemalan fauna. As a matter of fact, however, there is no definite limit between 
these zoo-geographical regions, no more than there are between the North- and 
South-European, and the Palearctic and Oriental or Indian region. The main 
divisions used in zoo-geography, radiating from one centre and extending their dominion 
until they meet another, with which it would intermingle only in a comparatively 
narrow border-zone, do not really exist; but each of them, the Nearctic as well as 
the Neotropical, or any other, includes an abstract compound of families and genera of 
very different geographical extension, all having their peculiar conditions of life, and 
therefore also succeeding more or less in the common endeavour of extending their 
habitat. For instance, a considerable part of the Nearctic fauna is formed of families, 
genera, and even species which live also in Northern Europe and Northern Asia: it is 
the circumboreal fauna, crossing the Northern Atlantic and the Northern Pacific, and 
prevailing not only in the desolate Arctic regions, but also in the forest- and lake- 
countries of British North America and a portion of the United States (e.g., Rangifer, 
Alce, and a Bison, the Mustelide and Arvicolide, the Tetraonide, Limnea stagnalis, 
Margaritana, &c.). But another essential part of the Nearctic fauna is formed of 
exclusively American families or genera, such as Didelphys, Procyon, the Sylvicolide, 
and the Strepomatide. Likewise within the Palearctic division we find, not only in 
its southern provinces, but even in Scandinavia and European Russia, families and 
genera which bear witness of a fauna limited to Europe and Africa, e. g. Hrinaceus and 
the Myoxidee. In the same manner, within the limits cf Mexico and Central America 
the various faunas are intermingled over the greater part of the region: the Nearctic 
represented by Strobi/a and Polygyra; that of New Mexico and Texas by Holospira 
and Praticolella; that of California and Peru by some Bulimulide ; the Caribbean by 
Choanopoma, Chondropoma, and Ortalichus; that of the South-American continent 
by Glabaris; and the Mexican and Central-American by Glandina, Streptostyla, 
and Kucalodium. 
As regards the elevation of the localities above the sea, I have noted what was 
available under each species in the body of the work ; for special particulars concerning 
