THE WIIITEFISHES OF NORTH AMERICA. 



287 



The measurements given in the descriptions in this paper are propor- 

 tional. il I lead 4" means that the length of the head is contained 4 times 

 in the length of the fish, measured from the tip of the snout to the base 

 of the tail or caudal fin. " Depth 5" means the greatest depth of the 

 body is contained 5 times in the same length. The lengths of the eye, 

 snout, maxillary, and mandible are compared with the length of the 

 head; thus "eye 5" means the diameter of the eye (not the orbit) is con- 

 tained 5 times in the length of the head. The snout is measured from 

 its tip to the anterior rim of the orbit. The maxillary is the long, 

 usually somewhat ovate, bone on side of upper jaw; the mandible is the 

 bone forming the side of the lower jaw. The length of the maxillary 

 and mandible is important. 



The gillrakers of the first gill-arch afford one of the best characters 

 for distinguishing the species of this group. The accompanying figure 

 shows the first gill-arch of the bloater or longjaw. From this it may 

 be seen that there are, in this case, 15 gillrakers on the upper or short 

 limb of the arch, and 31 on the long limb; the length of the longest is 

 contained 1A times in the diameter of the eye. These three facts are 

 ordinarily stated thus: Gillrakers 

 15+31, the longest 1| in eye. 



The number of scales is also im- 

 portant. "Scales 8-77-7" means 

 that there are 7 rows of scales in a 

 vertical series between the base of . 

 the dorsal fin and the lateral line, 

 or 8 including the scale in the lateral 

 line ; that there are 77 scales in a 

 longitudinal series counted along 

 the side from the gill-opening to the 

 base of the caudal fin, and that there are 7 scales between the lateral 

 line and the base of the ventral fin. The lateral line is the series of 

 modified scales along the middle of the side. 



The total number of species and subspecies of whitefishes recognized 

 by us as at present known from the waters of America is 20, at least 

 10 of which are known to inhabit the basin of the Great Lakes. The 

 whitefishes and lake herrings are now regarded by most students of 

 American fishes as belonging to one or the other of two rather closely 

 related genera, Argyrosomus and Coregonus, which may be distinguished 

 as follows: 



a. Mouth rather small, maxillary short and hroad; inaudible short; lower jaw 

 usually included and overhung by the more or less projecting snout; pre- 

 maxillaries broad, with the cutting edge nearly vertical or directed backward; 

 gillrakers on the first arch few and short, usually fewer than 30. . .COREGONUS. 



aa. Mouth rather large, maxillary long and narrow; mandible long; lower jaw 

 usually more or less projecting beyond the upper; premaxillaries with the 

 cutting edge nearly horizontal and directed forward; gillrakers on the lirst 

 arch long and numerous, usually more than 35 Argyrosomus. 



Gill-arch <>!' tb<- Longjaw. 



