XX INTRODUCTION. 
Tryonia, of the Lake of Peten, the other species of this genus being Californian. 
Unio, generally, as distinct from the South-American Diplodon (in the sense of 
y. Ihering and Simpson), more especially the subdivisions Crenodonta, Lampsilis, 
Elliptio, and Metaptera. The genus is much better represented in Mexico and 
Guatemala than in the more southern countries ; of the subdivisions mentioned, 
Elliptio only is represented in Nicaragua and in Panama, and by a single 
species, no Unio being known from Costa Rica. 
Anodonta, in the more restricted sense, and Patularia, both reaching no farther 
south than N. Guatemala and Yucatan. 
It is very remarkable, however, that the family Strepomatide (Pleuroceridz), so 
widely distributed in North America, from Lake Winnipeg and Montreal to Florida 
and Texas, and occurring also in California, has no known representative within our 
limits, not even in the northern parts of Mexico, Pachychilus schiedeanus having 
been proved by Troschel’s examination of the radula to be a Pachychilus and not 
a Gontobasis. 
FRESHWATER SHELLS SHOWING A SOUTH-AMERICAN AND ANTILLEAN AFFINITY. 
The subgenera Menetus and Spiralina of Planorbis, and the subgenus Stenophysa of 
Physa, each of which is well represented in the West-Indian Islands and on 
the continent of South America, all three ranging from Mexico to Nicaragua, 
the last even to Panama. 
Ampullaria, including the subgenus Ceratodes. Ampullaria, s. str., 1s represented 
in the Southern States of North America ( Georgia and Florida) by one species 
only. Within our limits it seems to be absent from the elevated tableland 
of Central Mexico, ranging from the southern parts of Mexico to Panama. 
In Guatemala and Honduras the species are numerous and very little inferior 
in size to those of the continent of South America, whereas the genus is 
somewhat poorly represented in the West-Indian Islands. Ceratodes belongs 
essentially to the South-American continent, exceeding its limits only in the 
adjacent islands of Tobago and Trinidad, and in Costa Rica, if the latter record 
is reliable (cf. p. 425). 
Semisinus, a Caribbean and South-American genus, extending within our limits 
from N. Guatemala to Costa Rica. 
Neritina, subgenus Nerite@a, including also Caribbean and South-American forms, 
