Food Belations of Fresh-Wafov Fishes. 505 



The teeth of our fresh-water fishes are always pointed and 

 acute, there being no examples of pavement teeth or cutting 

 incisors among them, such as are found in several marine 

 forms, nor are there any instances of either jaw being toothed 

 and the other not. The evanescent teeth of the young of 

 several species which become toothless when mature, are some- 

 times to be understood as rudiments, as in the shovel fish, and 

 sometimes as related to the early food, as in the white-fish and 

 the gizzard shad. 



The gill-rakers of fishes vary widely in number, length, and 

 usefulness, but are as important and significant as any other 

 part of the feeding apparatus. As they oppose the only 

 obstacle to the escape through the gill slit, of objects which 

 enter the mouth with the water of respiration, they set the 

 minimum of size for objects of the fishes' food, the only excep- 

 tion to this rule being afEorded by the few fishes which 

 swallow mud with little or no discrimination. 



They are usually arranged in two rows on each gill arch, 

 with frequently one also on the pharyngeal, behind the last gill 

 slit. Occasionally only one row is developed on each gill (lake 

 "herring"), and commonly the second row, if present, is less 

 prominent than the first. The shovel fishes are, however, an 

 exception to this latter statement, for in them both rows are 

 equally and remarkably developed. As the anterior rakers 

 guard the relatively large passage-way between the foremost 

 gill and the opercle, while the other rows merely prevent the 

 escape of objects between the several pairs of gills, the anterior 

 row is almost invariably longer than the remaining series. The 

 shovel fish and the gizzard shad are exceptions. The rakers of 

 this row are commonly longest in the middle of the arch, 

 shortening toward each end; but the particulars of this disposi- 

 tion depend on the length and shape of the arch and the con- 

 cavity of the inner surface of the opercle. In the gizzard 

 shad, however, the short but very numerous and fine gill-rakers 

 project in a nearly horizontal direction. 



The gill-rakers, when short and ineffective, are often armed 

 with minute denticles, variously arranged, but are never 

 branched or pinnate. In several of the sucker family, the 

 rakers of the lower horizontal arm of the arch are represented 



