INTRODUCTION. XXV 
numerous Costa-Rican forms I am indebted to H. Pittier and P. Biolley. But I dare 
not draw general conclusions, which might be disproved by the next observing traveller. 
The central Tableland of Mexico, without manifest drainage either to the Atlantic or 
to the Pacific, is treated in the enumeration of localities in the body of the work, and 
in the preceding Tables, as a distinct division. This region appears to be essentially 
poorer in Molluscs than the adjacent Eastern slope, the State of Vera Cruz, especially 
in Cyclophoridze and Cyclostomide, as well as in Omphalina, among the land-shells; 
Aplecta, Stenophysa, Ampullaria, Neritina, Glabaris, and Polymesoda, among the 
freshwater shells, are wanting in Central Mexico, while they are present on the 
Western and Eastern slopes: the number of freshwater shells is also remarkably 
smaller, especially in the genera Unio and Anodonta; Planorbis, on the contrary, and 
the subgenus Alampetis of Physa are, however, well represented: among the land-shells, 
Helicina, Glandina, Polygyra, Otostomus, Eucalodium, Holospira, and Succinea are 
comparatively numerous. In Guatemala, Nicaragua, and Costa Rica I have also 
distinguished for the land-shells a central region, as distinct from the Pacific 
and Atlantic slopes: in Guatemala it comprises the metamorphic formations of the 
upper districts drained by the River Usumacinta and the upper half of that of the 
Rio Grande or Motagua River; in Nicaragua it corresponds to the basin of the Lake of 
that name, draining to the Atlantic, although nearer geographically to the Pacific; and 
in Costa Rica it comprises the elevated regions in the vicinity of the capital, San José, 
1200 metres and more above the sea. The relationship of the fauna of these central 
parts to that of the neighbouring Pacific and Atlantic provinces will be seen 
particularized in the Tables: generally, it may be mentioned that Helicina, Glandina, 
Streptostyla, Otostomus, and Succinea are more or less well represented on the central 
plateaux, as in Central Mexico, and the Cyclophoridee and Cyclostomatidee very scarce. 
The species hitherto known to occur in the highest regions (2400-3500 metres, or 
about 8000-11,600 feet) on the central Tableland of Mexico and Guatemala belong 
to the genera Helicina (see infra, pp. 33, 37, 603), Glandina (pp. 54, 73), and Otostomus 
(pp. 208, 210) *. 
* Fischer and Crosse (Miss. Sci. Mex., Moll. ii. p. 677) mention correctly in this respect Helicina and 
Glandina ; but they go too far in stating that in South America, as a difference from the fauna of Mexico and 
Guatemala, the land-shells of the most elevated regions are Bulimulus ; for Otostomus sulcosus (Bulimulus, in 
the sense of Fischer and Crosse) reaches in Mexico the height of 3200 metres (see infra, p. 208), and 
O. ghiesbreghti, in Guatemala, reaches 2600-2900 metres, which is a much greater elevation than that 
known for Glandina (2250-2400 metres), but less than that reached by Helicina (3500-3900 metres). 
BIOL. CENTR.-AMER., Terr. and Fluviat. Mollusca, May 1901. d 
