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BULLETIN OF THE BUREAU OF FISHERIES 



and Huron have relatively larger eyes, and those in Superior have relatively smaller 

 eyes than the form in Michigan ; in Jioyi the eye is proportionally larger in the Superior 

 and Nipigon forms and smaller in the Ontario race; in artedi the manitoulinus form 

 has a relatively larger eye than the other subspecies of artedi. 



Teeth 



Vestigial teeth have been found in the forms of all species except quadrilaterale. 

 They are present more or less regularly on the premaxillaries, the palatines, the 

 mandible, and on the tongue, and are least in evidence in the larger individuals of 

 each species. Those on the tongue appear to be retained longest. The absence of 

 teeth then serves to separate Prosopium from the other genera. 



Branchiostegal Membrane and Rays 



The outline of this membrane and the number of its rays have no taxonomic value 

 in this group, except that Prosopium differs from the other two groups in both of 

 these respects. In the latter the membrane is saber-shaped and usually contains 

 8 to 10 rays. In Prosopium it is trapezoidal in outline, and there are only 7 or 8 rays. 



Gill Rakers 



The number of gill rakers is of great systematic importance. Leucichthyshas the 

 most and longest rakers and Prosopium the fewest and shortest. The range of the 

 latter overlaps that of none of the other species; that of Coregonus overlaps but 

 rarely the range of any species other than johannae. 



Leucichthys may be divided into three groups: (1) Gill rakers on the first arch 

 commonly less than 33 (johannse); (2) usually more than 32 and usually less than 44 

 {alpense, zenithicus, reighardi, and Tciyi (except in Ontario); (3) usually more than 43 

 (nigripinnis except in Superior, artedi and nipigon). The range of hoyi and Ontario 

 Tciyi is about intermediate between 2 and 3. Superior nigripinnis falls about in Group 

 2. As has been indicated above, there may be some variation in the number of gill 

 rakers on the first branchial arch within a species. The alpense of Huron have some- 

 what fewer gill rakers than those of Michigan; they are somewhat fewer in the 

 zenithicus of Nipigon and Huron than in those of other lakes; they are somewhat 

 fewer in the reighardi of Nipigon ; in the nigripinnis of Nipigon they are more numer- 

 ous, and they are fewer in the Superior form; they are more numerous in the Tciyi of 

 Ontario; they are more numerous in the Tioyi of Nipigon and Ontario. 



Within the species of a lake no striking variation tendencies have been noted in 

 any of the races. It is noticeable, however, that in the case of alpense and zenithicus 

 small fish have somewhat fewer gill rakers, due, no doubt, to the imperfect develop 

 ment of those on the ends of the arches. 



Nares 



The structure of the nares is a distinctive feature in Prosopium. In Leucichthys 

 and Coregonus the anterior opening is through a short tube obliquely truncated 

 toward the front, and a rather broad membranous flap is present at the anterior end 

 of the posterior opening. In Prosopium this flap is wanting. (See fig. 27.) 



