Xxil INTRODUCTION. 
and the same remark applies to the Mollusca of the States of Coahuila, Nuevo Leon, 
and Tamaulipas, as compared with those of Texas. Streptostyla novoleonis is the first 
species to appear of the bright tropical fauna of Central America, and at Mazatlan an 
Ortalichus is a conspicuous land-snail. On the elevated tableland of Central Mexico 
the somewhat northern Polygyra, Holospira, and Helix humboldtiana are associated 
with the subtropical Glandina and the more tropical Otostomus and Hucalodium ; and 
in the State of Vera Cruz, not only in its hot lowlands, but also in the temperate hill- 
region of Jalapa, Mirador, Cordova, and Orizaba, the Molluscan fauna approaches 
still nearer to that of Central America*. Among the freshwater shells, Alampetis, 
Oxymelania, and Potamanax may be named as rather characteristic. Vivipara and 
Valvata are North-American forms not reaching farther south than Mexico. Among 
the Bivalves, Anodonte of South-American affinity (Huryanodon and Pachyanodon) 
make their first appearance in the southern part of Mexico, both east and west. 
2. GUATEMALA, with Tapasco and CuHIApas (S.E. Mexico in our Tables), Yucatan, 
and Betize (British Honduras), from the Isthmus of Tehuantepec to the constriction 
between Istapa and the Golfo Dulce. This tract of land possesses the most typical 
Central-American forms: several Cyclophoride and Cyclostomide, Tomocyclus, 
Helicina, and Glandina in moderate numbers, many Streptostyle, and a few Omphaline; 
Trichodiscina and Praticolella are still present, Lysinoé is comparatively rich, and the 
group of Helix costaricensis is already represented by H. trigonostoma with its many 
varieties; Polygyra inhabits the lower districts of the Usumacinta, and Holospira is to 
be found in Yucatan; Ortalichus, well-developed Otostomus, Eucalodiuwm, and Celo- 
centrum are richer here than elsewhere; Lpirobia, Macroceramus, Pseudosubulina, 
and Leptinaria, each with several species; and Simpulopsis takes the place of the 
exclusively Mexican Xanthonyx. The characteristic Mexican forms are still visible, as 
are also most of the prevailing West-Indian and South-American ones. Among the 
freshwater shells numerous species of Unio of North-American affinities are present, 
but the large-sized Ampullarie and Pachychili give a tropical character to the 
whole. 
3. Honpuras, Satvapor, Nicaragua, and Mosquito Coast, from the re-entering 
* Fischer and Crosse (Miss. Sci. Mex., Moll. ii. p. 673) were therefore justified in uniting the whole 
Atlantic slope of Mexico, from Tamaulipas in the north, with Yucatan, British Honduras, and Guatemala, 
into one geographical subregion. 
