21 THE FRESH- WATER FISHKS OF KUIIOPK. 



above the extremity of the snout, which is occupied by the jaws. Both 

 jaws are evenly covered with broad bands of teeth, like short bristles, and 

 the vomer and palatine bones both carry similar teeth. The tongue is large 

 and fleshy, and is smooth, like the middle of the palate. There are seven 

 branchiostegal rays on each side; these are attached to the hyoid bones, 

 termed cerato-hyals and epi-hyals, and are seen below the operculum. Four 

 branchial arches, which support the gills, are attached to the skull. These 

 arches consist of an upper part, termed epi-branchial, and a lower cerato- 

 branchial portion. Both bones of the branchial arch carry bony processes, 

 termed gill-rakers, which are directed towards the mouth, and defend the 

 entrance to the branchial clefts. The gills are attached to grooves on the 



Fig. 10. - PEKCA FLVVIATILIS 



outer side of the epi-branchial and cerato-branehial bones. The gills are 

 comb-shaped, and greatly developed in the young animal. 



The opercular bones give characters which define the species. The margins 

 of the pre-opercular bone form a right angle or an obtuse angle, the upper 

 or vertical part of which is very finely denticulated, and the lower edge 

 more coarsely serrated, with the serrations directed forward ; the first process 

 is generally strongest and largest. The opercular bone bears scales only 

 on its upper half, and ends posteriorly in a long spike, under which are seen 

 one or two other serrations. The blunter point of the sub-operculum ex- 

 tends farther backward, and the lower third of its margin is finely and 

 evenly serrated, though the serrations are scarcely distinguishable in the 

 fresh fish. There are minute serrations on the inferior border of the inter- 

 operculum. Scales extend over the cheeks and the sub-operculum ; but the 

 fins, the entire upper part of the head, the sub-orbital ring, and the inter- 

 operculum are naked. 



The first dorsal fin is placed at a distance behind the snout equal to the 



