328 



BULLETIN OF THE BUREAU OF FISHERIES 



DISCUSSION OF VARIABILITY 

 Interspecific Variations 



From the foregoing analyses it is evident that while there are adequate characters 

 to distinguish between the three genera, such as shape of the head and premaxil- 

 laries, size of maxillary and mandible, teeth, branchiostegal membrane, number and 

 length of gill rakers, structure of nares, pearl organs, etc., there are comparatively few 

 characters that can be used to separate the species within a group. It has been 

 shown that the number of rakers on the first branchial arch is of greatest value in 

 distinguishing the specific forms. Separated by this character, most of the coregonid 

 forms fall into four groups: (1) Rakers usually less than 20 (quadrilaterale) ; (2) rakers 

 usually more than 20 and less than 33 (clupeaformis and iohannse); (3) rakers usually 

 more than 32 and less than 44 (alpenae, zenithicus, reighardi, Tciyi, and nigripinnis of 

 Superior) ; (4) rakers usually over 43 (nigripinnis except Superior, artedi and nipigon.) 



Both quadrilaterale and clupeaformis are distinct from the other forms in the Great 

 Lakes and from each other, and each is the sole representative of a genus. It is 

 unnecessary, therefore, to consider them in the subsequent discussion. 



There remain for consideration the species of Leucichthys, which have hardly 

 any other character than gill rakers by which the species, wherever they may occur, 

 may be separated. Within a lake a few other characters may be of use to distinguish 

 one or two species from the rest (see analyses on pp. 335 to 339), but even these are 

 not very constant, and each taken alone certainly could not be relied upon. The 

 few characters that are fairly constant for each species are repeated below: 



1. Body contour. — The form of the body is, in general, fairly constant. In 

 alpenae, zenithicus, reighardi, hoyi, artedi, and nipigon the body outline, as seen from 

 the side, is generally elliptical; in johannse, nigripinnis, and Tciyi it is more or less 

 ovate as a rule. 



2. Length oj the lower jaw. — The mandible in alpense usually projects beyond 

 the upper jaw, while in zenithicus and reighardi it is generally included within the 

 upper. The other species (except Tciyi and artedi) that normally have equal jaws 

 occasionally may have the lower jaw longer or shorter than the upper. Artedi most 

 often has the lower jaw shorter than the upper, while in Tciyi it is usually a little 

 longer. 



3. Length oj the maxillary. — For artedi (except the form manitoulinus) the 

 maxillary usually is contained more than 2.6 times in the head length; for the rest 

 (excepting typical reighardi, which is intermediate) it usually is contained less than 

 2.7 times. 



4. Pigmentation oj the maxillary. — This character has more or less systematic 

 value. In johannx and alpense the maxillary usually is immaculate; in all the rest 

 it most often is more or less pigmented. 



The conclusion is unavoidable that those characters that are of greatest impor- 

 tance in the taxonomy of other groups of fishes, such as body proportions, number 

 of scales, fin rays, teeth, etc., are not of prime taxonomic value for the Coregonidse. 



Intraspecific Variations 



Analysis of the Great Lakes coregonids shows, in each of the 11 species, a wide 

 range of variation in all the characters that are of taxonomic importance, although 



