310 Percoidea, or Perch-like Fishes 



and especially abundant at the confluence of rivers. Gymno- 

 cephalus schrcetzer of the Danube has the head still more cav- 

 ernous. Percarina demidofft 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 pseudobranchiae better 



FIG. 243. The Zingel, Zingel zingel (Linnaeus). 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: Etheostominae. 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 Percina. 



