Food Relations of Fresh- Water Fishes. 481 



tant part of their food in the molluscan forms abundant in the 

 waters which they themselves most frequent. The class as a 

 whole makes about one fourth of the food of the dog-fish and the 

 sheepshead, — taking the latter as they come, half-grown and 

 adults together, — about half that of the cylindrical suckers, — ris- 

 ing to sixty per cent, in the red horse,' — and a considerable ratio 

 (fourteen to sixteen per cent.) of the food of the perch, the 

 common catfishes (Amiurusand Ictalurus). the small-mouthed 

 sunfishes, the top minnows, and the shiner (Notemigonus). 

 Notwithstanding the abundance of the fresh water clams 

 or river mussels (Unio and Anodonta), only a single 

 river fish is especially adapted to their destruction, viz., 

 the white perch or sheepshead; and this species derives, on 

 the whole, a larger part of its food from univalve than from 

 bivalve mollusks, the former being eaten especially by half- 

 grown specimens, and the latter being the chief dependence of 

 the adults. 



The ability of the catfishes to tear the less powerful 

 clams from their shells has been especially discussed in an- 

 other paper* containing the details of the food of the family. 

 Even the very young Unios were rarely encountered in the food 

 of fishes, my notes recording their presence in only three sun- 

 fishes, a brook silversides, and a perch. Large clams were 

 eaten freely by the full-grown sheepshead — whose enormous 

 and powerful pharyngeal jaws with their solid pavement teeth 

 are adapted to crushing the shells of mollusks — and by 

 the bull-heads (Amiurus), especially the marbled cat.' The 

 small and thin-shelled Spha^riums are much more frequent 

 objects in the food of moUusk-eating fishes than are the Unios. 

 This genus alone made twenty-nine per cent, of the food of 

 our one hundred and seven specimens of the sucker family, and 

 nineteen per cent, of that of a dozen dog-fishes. Among the 

 suckers it was eaten greedily by both the cylindrical and the 

 deep-bodied species, although somewhat more freely by the 

 former. Even the river carp,'* with its weak pharyngeal jaws 

 and delicate teeth, finds these sufficient to crush the shells of 

 Sphairiura, and our nineteen specimens had obtained about 



• Bull. 111. St. Lab. Nat. Hist., Vol. II., pp. 457, 45S. 

 i.'Moxostoma. - Amiurus marmoratus. ^ Carpiodes. 



