(4) =The Brazilian Sub-region.=--This immense district is very little known, except in the south, and it is consequently impossible to give any satisfactory account of its Mollusca. It is possible that eventually it will be found that it falls into provinces which correspond more or less to (_a_) the Amazon basin; (_b_) the mountainous district in the east, drained by the Tocantins and the San Francisco; (_c_) the Parana basin in the south central district; and (_d_) the Argentine or Pampas district in the extreme south. But at present the data are insufficient to establish any such subdivisions, whose existence, if proved, would have an important bearing on the problem of the coalescence of S. America into its present form.[379] The Agnatha are represented by _Streptaxis_ alone (17 sp.). _Helix_ is rare, but includes the peculiar _Polygyratia_ (Fig. 150 A, p. 246), while _Labyrinthus_ (2 sp.), _Solaropsis_ (5 sp.), and _Systrophia_ are common with the Colombian Sub-region, and _Oxychona_ (4 sp.) with the Central American. _Bulimus_ has in all 36 species, the sub-genera _Pachyotus_ (Fig. 233) and _Strophochilus_ being peculiar. _Bulimulus_, though not so abundant as in Peru and Ecuador, has about 60 species, of which _Navicula_ (Fig. 235) is the most remarkable group. _Megaspira_ is peculiar. _Orthalicus_ has only 4 species, while _Tomigerus_ (4 sp.) and _Anostoma_ (3 sp.) are common with Venezuela. Land operculates are scarce, and appear to include only _Neocyclotus_, _Cyclophorus_, and _Helicina_. In Argentina, which may probably rank as a separate province, the tropical forms greatly decrease, _Streptaxis_ being reduced to 2 species, and _Bulimus_ and _Bulimulus_ together to 40, while _Orthalicus_, the great Helices, and the land operculates disappear altogether. _Odontostomus_ (Fig. 236), a genus of the Pupidae, is abundant in the northern part of the province. Two or three species of _Chilina_ occur. [Illustration: FIG. 235.--_Bulimulus_ (_Navicula_) _navicula_ Wagn., Brazil.] [Illustration: FIG. 236.--_Odontostomus pantagruelinus_ Moric., S. Brazil. × ½.]