INTRODUCTION. 
More than ten years ago, chiefly at the invitation of my friend Dr. P. L. Sclater, 
I undertook the task of compiling the Volume on the Land and Freshwater Mollusca 
for the ‘ Biologia Centrali-Americana.’ It is almost needless to say that the work has 
taken me much longer than I anticipated, notwithstanding the previous labours of 
P. Fischer and H. Crosse, H. Strebel, and myself on the Molluscan fauna of the 
same region. Many species obtained by our Editors, as well as others in the Berlin 
Museum, or sent to me by various friends and collectors, required further examination ; 
and when this Volume was commenced the ‘‘ Mollusques ” of the ‘ Mission Scientifique 
en Mexique et dans l’Amérique Centrale,’ and the ‘ Beitrag zur Kenntniss der Fauna 
mexikanischer Land- und Stisswasser-Conchylien,’ were only in course of publication. 
Official occupation, too, at the Berlin Museum took up nearly all my time, so that 
I could bestow very little more than my leisure hours on this work; but I have had 
at my disposal the rich collection of shells and the valuable library belonging to the 
‘Museum fiir Naturkunde.” 
The systematic arrangement of the Mexican and Central-American forms having 
been dealt with satisfactorily by the above-mentioned authors, my chief object has 
been to collate all the published papers, as well as various unpublished MS. notes, 
on the subject, in order to obtain a sound foundation for remarks on zoo-geographical 
distribution. For this purpose the known localities for each species have been grouped 
or arranged under different headings, chosen with regard to the political frontiers and 
the eastern and western slopes of the central portion of each country belonging to the 
region under investigation; the scheme adopted, it is true, has been slightly varied, 
or rather corrected, since the publication of the first portion of the work, but it will 
be, I think, easily understood in every case. 
