GREAT LAKES COREGONIDS 



325 



illary, in proportion to the head, is relatively shortest in artedi and reighardi, the 

 figures are not exclusive. Coregonus has a shorter maxillary than most forms of 

 Leucichthys, but the range overlaps slightly. Prosopium has relatively the shortest 

 maxillary of all, and its figures overlap but little those of Coregonus. The usual 

 value for H/M in typical artedi and the albus form is 2.7 to 2.9; in typical reighardi } 

 2.6 to 2.8; in the other forms of Leucichthys, 2.3 to 2.6; in Coregonus, 3.1 to 3.4; 

 and in Prosopium, 4.0 to 4.2. 



The maxillary length also varies within the species, so that in zenithicus it is 

 somewhat shorter in Michigan and Huron ; in reighardi it is longer in Superior and 

 Nipigon than in Michigan and somewhat shorter in Ontario; in hoyi it is longer in 

 Superior and Nipigon. In the species of one lake it also varies. Thus, reighardi of 

 northern Lake Michigan has, on the average, a proportionally longer maxillary than 

 that of the south, and the manitoulinus race of artedi has a much longer maxillary 

 than the typical race. 



Mandible 



In Leucichthys' the lower jaw is approximately equal to the upper; in the 

 other genera it is always distinctly shorter. In the case of reighardi and zenithicus 

 it is usually (in artedi often) somewhat shorter than the upper; in alpense and Tciyi 

 it is usually longer than the upper; in hoyi it is seldom shorter; in the others it is 

 variable, though most often about equal. It also varies in position within the species, 

 as evidenced by the fact that the forms of zenithicus in Michigan and Huron more 

 often have the mandible included; the dymondi race of reighardi often has the man- 

 dible not so conspicuously included; the cyanopterus form of nigripinnis has the 

 lower jaw shorter than the upper more frequently than the other nigripinnis races; 

 and the deep-water hoyi of Huron and the hoyi of Nipigon and Ontario seem to have 

 longer jaws than their relatives elsewhere. 



Within the group Leucichthys there is nothing distinctive about its shape, 

 degree of development, or relative length compared with that of the head. The 

 degree of bony development, however, is most pronounced in alpense and probably 

 least in reighardi. In Leucichthys the value obtained by dividing the mandible 

 into the head never is more than 2.3 and may be as low as 1.7. In Coregonus the 

 value usually is 2.4 to 2.7; in Prosopium it is 2.7 to 3.1. 



Eye 



The size of the eye varies with the age of the individual and consequently is of 

 ready systematic value only in forms of comparable size or state of development. 

 The values obtained by dividing the eyeball into the head are given in Table 9, 

 first for the adults of the larger forms and then, so far as possible, for their young, 

 for comparison with the former and with the smaller species of the group. As is 

 usual in fishes, the eye appears to be relatively larger in the young than in the adult 

 and the values are not very different for any of the species. 



The eye size also varies within the species. The alpense of Huron seems to have 

 a proportionally smaller eye than that of Michigan; zenithicus of Nipigon seems to 

 have a somewhat larger eye than that of other lakes; the reighardi of Ontario and 

 Nipigon seem to have a smaller eye than those from other lakes ; nigripinnis in Nipigon 



