Allied to the perch, but long, slender, big-mouthed, and voracious, is the group of pike perches, found in eastern America and Europe. The wall-eye, or glass-eye (_Stizostedion vitreum_), is the largest of this tribe, reaching a weight of ten to twenty pounds. It is found throughout the region east of the Missouri in the large streams and ponds, an excellent food-fish, with white, flaky flesh and in the north a game fish of high rank. The common names refer to the large glassy eye, concerning which Dr. Goode quotes from some "ardent admirer" these words: "Look at this beautiful fish, as symmetrical in form as the salmon. Not a fault in his make-up, not a scale disturbed, every fin perfect, tail clean-cut, and his great, big wall-eyes stand out with that life-like glare so characteristic of the fish." [Illustration: 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. [Illustration: 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