Percoidea, or Perch-like Fishes §25 
and especially abundant at the confluence of rivers. Gymno- 
cephalus schretzer of the Danube has the head still more cav- 
ernous. 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. Ztngel differs from Perctna mainly in having seven 
instead of six branchiostegals and the pseudobranchie better 
Fic. 412.,—The Zingel, Zingel zingel (Linneus). Danube River. (After Seelye.) 
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 char- 
acters 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. 
The Darters: Etheostomine.— Of the darters, or etheosto- 
mine perches, over fifty species are known, all confined to the 
streams of the region bounded by Quebec, Assiniboia, Colo- 
rado, and Nuevo Leon. All are small fishes and some of them 
minute, and some are the most brilliantly colored of all fresh- 
water fishes of any region, the most ornate belonging to the 
large genus called Etheostoma. The largest species, the most 
primitive because most like the perch, belong to the genus Perctna, 
