Besides these external elements, the region is rich in indigenous genera. Central America is remarkable for an immense number of large carnivorous Mollusca possessing shells. There are 49 species of _Glandina_, the bulk of which occur in eastern and southern Mexico; 36 of _Streptostyla_ (S.E. Mexico and Guatemala, only 1 species reaching Venezuela and another Peru); 5 of _Salasiella_, 2 of _Petenia_, and 1 of _Strebelia_; the last three genera being peculiar. _Streptaxis_, fairly common in S. America, does not occur. _Velifera_ and _Cryptostracon_, two remarkable slug-like forms, each with a single species, are peculiar to Costa Rica. Among the especial peculiarities of the region are the giant forms belonging to the Cylindrellidae, which are known as _Holospira_, _Eucalodium_, and _Coelocentrum_ (Fig. 232). They are almost entirely peculiar to Mexico, only 7 out of a total of 33 reaching south of that district, and only 1 not occurring in it at all. [Illustration: FIG. 232.--Examples of characteristic Mexican Mollusca: =A=, _Coelocentrum turris_ Pfr.; =B=, _Streptostyla Delattrei_ Pfr.] The land operculates are but scanty. _Tomocyclus_ and _Amphicyclotus_ are peculiar, and _Schasicheila_, a form of _Helicina_, occurs elsewhere only in the Bahamas. _Ceres_ (see Fig. 18, C, p. 21) and _Proserpinella_, two remarkable forms of non-operculate Helicinidae (compare the Chinese _Heudeia_), are quite peculiar. _Pachychilus_, one of the characteristic fresh-water genera, belongs to the S. American (Melaniidae) type, not to the N. American (Pleuroceridae). Among the fresh-water Pulmonata, the _Aplecta_ are remarkable for their great size and beauty. In the accompanying table “Mexico” is to be taken as including the region from the United States border up to and including the isthmus of Tehuantepec, and “Central America” as the whole region south of that point. _Land Mollusca of Central America_ Mexico only. Central Common to America both. only.