INTRODUCTION, XXxl 
extends from Mexico to Nicaragua, but is absent from the elevated tableland 
of Central Mexico *. 
Glabaris (as understood by Simpson), or the Anodonta with deep triangular sinulus 
(p. 525), a genus extending within our limits from S.E. Mexico to Panama. 
Mycetopus, a genus extending as far north as S. Guatemala. 
Eupera, a genus extending as far north as Yucatan. 
There are scarcely any Mexican or Central-American species more nearly allied to 
the West-Indian forms than to those of South America, probably because in all islands, 
unless they are of very large extent (such as Madagascar, Borneo, and Australia), the 
freshwater fauna is generally poor, We may mention the great resemblance of 
Unio yzabalensis, of Eastern Guatemala and Honduras, to the Cuban U. scamnatus. 
Amnicola coronata is widely spread in the West-Indian Islands, and also in Central 
America from Vera Cruz to Nicaragua, but on the continent of South America it is 
confined to Colombia and Venezuela. Menetus, Spiralina, and Stenophysa are equally 
characteristic of the freshwater fauna of the West-Indian Islands, and Gundlachia also 
inhabits Cuba, California, and the Potomac. 
From the above analysis it is clear that the Terrestrial and Freshwater Mollusca of 
Mexico and Central America, as a whole, are not altogether peculiar, neither can they 
be termed North-American nor South-American ; and as regards their relationship with 
the West-Indian fauna, no definite conclusion can be arrived at. But if one or two 
genera only are taken into consideration, it would be easy to suggest an affinity with 
the fauna of any of these main regions. 
On referring to a map, we find that the long stretch of country included within the 
scope of this work is narrowed in several places, so as to form four large tracts of land, 
as well as a small additional piece :— 
1. Mexico generally, to the Isthmus of Tehuantepec, broadly connected with the 
United States from the head of the Gulf of California to the mouth of the Rio 
Grande del Norte. Here, of course, the resemblance to the North-American fauna 
is the greatest, the species obtained in the districts explored by the ‘ Boundary 
Commission ”—Sonora, Arizona, Chihuahua, and New Mexico—being almost identical ; 
* On pages 472, &c., I have placed the locality Sayula urder the head of Central Mexico, but it would 
perhaps be more correctly assigned to S.W. Mexico. 
