Xvl INTRODUCTION. 
Arizona, but is wanting in Guatemala; Hpirobia inhabits the southern part 
of Eastern Mexico (State of Vera Cruz), extending southward to Kastern 
Guatemala *. 
Amongst the genera common to Central and South America and the West-Indian 
Islands, and absent from North America (with the exception of Chondropoma, which 
occurs in Florida), are Oyclotus (Neocyclotus), Cyclophorus (Amphicyclotus), Chondro- 
poma, and Cistula. Labyrinthus is peculiar to the southern part of Central America 
(from Nicaragua southward) and the northern half of South America. The species 
enumerated under the subgeneric names Praticolella, Lysinoé, and Oxychona are 
characteristic of several parts of Mexico, Guatemala, and Costa Rica; but their natural 
affinities have not yet been made out sufficiently well to show whether they bear 
any relationship to the fauna of Western North America or to that of the Andes of 
the southern continent. Opeas is circumtropical, and Succinea has a world-wide 
distribution. 
As regards the smaller forms, and those which are without external shell, it must 
be borne in mind that their distribution in Mexico and Central America, so far as at 
present known, shows the results of collecting in particular localities rather than true 
zoo-geographical data. The most fully explored district is, without doubt, the State 
of Vera Cruz (E. Mexico, of our Tables); then follows Guatemala, Costa Rica, 
Chiapas and Tabasco, Central Mexico and Mazatlan (N.W. Mexico), and the region 
bordering the United States frontier; the least-known districts are Salvador and the 
State of Panama. 
The more characteristic genera and subgenera of the land-shells may be erouped 
together to show their geographical relationship in the following manner :— 
A. Essentially Mexican and Central-American. 
a. Not extending to the greater part of North nor to South America. 
aa. Not represented in the West-Indian Islands. 
Tomocyclus, southward to Honduras; Streptostyla; Trichodiscina, southward to Costa Rica ; 
Praticolella, from Arkansas and Texas to Costa Rica; the group of Helix costaricensis 
* It has been recorded from Venezuela, but this statement requires confirmation. 
