INTRODUCTION. XXill 
angle of the Gulf of Honduras with the Golfo Dulce to the depression of the Lake of 
Nicaragua. Here a comparative impoverishment of the fauna commences: Eucalodium, 
Celocentrum, and Omphalina are absent; Polygyra is represented by one species only ; 
and the Cyclophoride, Glandina, Streptostyla, and Ortalichus diminish in number. 
Among the freshwater shells, the species of Unio are fewer in number and of smaller 
size; the true Anodonte of northern affinity have disappeared; and the subgenus 
Arotonaias is nearly, and Scolianodon quite, peculiar to the basin of the Lake of 
Nicaragua. 
4. Costa Rica, with Curriqur and Vreracuas, from the depression of Nicaragua to 
the Isthmus of Panama. The general features are about the same as in no. 3, but the 
number of known species is greater, for which we are indebted to the recent exertions 
of H. Pittier and P. Biolley; but the increase consists chiefly in small-sized repre- 
sentatives of more widely-distributed genera. ‘The impoverishment of conspicuous and 
characteristic forms still continues: the Cyclostomide and Tonocyclus are apparently 
absent, and Polygyra also; Helix ghiesbreghti is replaced by the group of H. costari- 
censis, including three species; the Cyclophoride, Glandina, Streptostyla, Trichodiscina, 
Ortalichus, and Bulimulus are well-represented; and new strictly South-American 
forms, like Labyrinthus and Solaropsis, make their appearance. Trichodiscina and the 
eroup of Helix costaricensis are the only subgenera which are not to be found in any 
part of South America. The total absence of Unionide in Costa Rica, if real and not 
simply due to imperfect research, may be attributed to the absence of large slowly- 
flowing rivers and great lakes. 
5. The small portion of the Stare or Panama east of the Isthmus. This region 
seems to have a fauna like that of the northern part of the South-American continent. 
I should have preferred the Isthmus as a limit in my Tables and for the whole work, 
but I have been constrained to adopt the political boundary of the State of Panama 
(which includes also the Isthmus of Darien), because authors give for many species 
simply “‘ Panama” as locality. These species can be enumerated safely as found within 
the State of Panama, but I have no means of knowing whether east or west of the 
deepest notch of the Isthmus. 
The facts stated show that neither the Isthmus of Tehuantepec nor that of Panama 
affords a satisfactory boundary-line between the North- and South-American fauna ; 
