XXIV INTRODUCTION. 
element of the fresh-water fish-fauna, but the generic types are comparatively few; 
some of the existing European genera are represented in Oligocene and Miocene deposits. 
‘The Nearctic Cyprinine number about 225 species, most of which belong to the 
Palearctic genus Leuciscus or to closely allied generic types, there being, perhaps, less 
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DisTRIBUTION oF CYPRININZ. 
diversity in this region than in any other. Consequently the generic distinctions are 
in great part based on very trivial characters, and slight differences in the form and 
number of the pharyngeal teeth are considered of importance. 
In America the Cyprinine range from British Columbia, Saskatchewan, and Quebec 
to the Rio Balsas in Southern Mexico. As in the Catostomine, few genera are 
common to both sides of the Rocky Mountains, except in the case of types specially 
adapted to mountain-streams. 
Of the 40 species included in the fauna of Mexico, 10 belong to six endemic genera, 
of which five, A/gansea (4 species), Malcula (1 species), Kvarra (2 species), Xystrosus 
(1 species), and Yuriria (1 species), are peculiar to the Lerma System and the other, 
Stypodon, contains a single species from Coahuila. Notropis, with about 100 species, 
all from rivers east of the Rocky Mountains and the Sierra Madre, is represented by 
13 in Mexico; 10 of these are from north of the Lerma and 3 from the Lerma System ; 
