FIG. 241.—Sauger, _Stizostedion canadense_ (Smith). Ecorse, Mich. ]
Similar to the wall-eye, but much smaller and more translucent in color, is the sauger, or sand-pike, of the Great Lakes and Northern rivers, _Stizostedion canadense_. This fish rarely exceeds fifteen inches in length, and as a food-fish it is of correspondingly less importance.
FIG. 242.—The Aspron, _Aspro asper_ (Linnæus). Rhone River. Family _Percidæ_. (After Seelye.) ]
The pike-perch, or zander, of central Europe, _Centropomus_ (or _Sandrus_) _lucioperca_, is an excellent game-fish, similar to the sauger, but larger, characterized technically by having the ventral fins closer together. Another species, _Centropomus volgensis_, in Russia, looks more like a perch than the other species do. _Sandroserrus_, a fossil pike-perch, occurs in the Pliocene. Another European fish related to the perch is the river ruff, or pope, _Acerina cernua_, which is a small fish with the form of a perch and with conspicuous mucous cavities in the skull. It is common throughout the north of Europe and especially abundant at the confluence of rivers. _Gymnocephalus schrætzer_ of the Danube has the head still more cavernous. _Percarina demidoffi_ of southern Russia is another dainty little fish of the general type of the perch. A fossil genus of this type called _Smerdis_ is numerously represented in the Miocene and later rocks. The aspron, _Aspro asper_, is a species like a darter found lying on the bottoms of swift rivers, especially the Rhone. The body is elongate, with the paired fins highly developed. _Zingel zingel_ is found in the Danube, as is also a third species called _Aspro streber_. In form and coloration these species greatly resemble the American darters, and the genus _Zingel_ is, perhaps, the ancestor of the entire group. _Zingel_ differs from _Percina_ mainly in having seven instead of six branchiostegals and the pseudobranchiæ better developed. The differences in these and other regards which distinguish the darters are features of degradation, and they are also no doubt of relatively recent acquisition. To this fact we may ascribe the difficulty in finding good generic characters within the group. Sharply defined genera occur where the intervening types are lost. The darter is one of the very latest products in the evolution of fishes.