Fishes . with theform of a perch and with conspicuous mucous cavities inthe skull. It is common throughout the north of Europe Percoidea, or Perch-like Fishes 525 and especially abtmdant at the confluence of rivers. Gymno-cephaliis schrcetzer of the Danube has the head still more cav-ernous. Per carina demidoffi of southern Russia is anotherdainty little fish of the general type of the perch. A fossilgenus of this type called Smerdis is numerously represented inthe Miocene and later rocks. The aspron, Aspro asper, isa species like a darter found lying on the bottoms of swift rivers,especially
Fishes . with theform of a perch and with conspicuous mucous cavities inthe skull. It is common throughout the north of Europe Percoidea, or Perch-like Fishes 525 and especially abtmdant at the confluence of rivers. Gymno-cephaliis schrcetzer of the Danube has the head still more cav-ernous. Per carina demidoffi of southern Russia is anotherdainty little fish of the general type of the perch. A fossilgenus of this type called Smerdis is numerously represented inthe Miocene and later rocks. The aspron, Aspro asper, isa species like a darter found lying on the bottoms of swift rivers,especially the Rhone. The body is elongate, with the pairedfins highly developed. Zingel zingel is found in the Danubeas is also a third species called Aspro streber. In form andcoloration these species greatly resemble the American darters,and the genus Zingel is, perhaps, the ancestor of the entiregroup. Zingel differs from Percina mainly in having seveninstead of six branchiostegals and the pseudobranchice better. Fig. 412.—The Zingel, Zingel zingel (Linnaeus). Danube River. (After Seelye.) developed. The differences in these and other regards whichdistinguish the darters are features of degradation, and theyare also no doubt of relatively recent acquisition. To thisfact we may ascribe the difficulty in finding good generic char-acters within the group. Sharply defined genera occur wherethe intervening types are lost. The darter is one of the verylatest products in the evolution of fishes. The Darters: Etheostominae. — Of the darters, or etheosto-mine perches, over fifty species are known, all confined to thestreams of the region bounded by Quebec, Assiniboia, Colo-rado, and Nuevo Leon. All are small fishes and some of themminute, and some are the most brilliantly colored of all fresh-water fishes of any region, the most ornate belonging to thelarge genus called Etheostoma. The largest species, the mostprimitive because most like the perch, belong to the genus Percina. 526 Percoide
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