GREAT LAKES COREGONIDS 



323 



zenithicus it seems smaller in Michigan and Huron and larger in Nipigon than in 

 Superior; in reighardi it is longer in Nipigon and Superior and shorter in Ontario than 

 in Michigan; in nigripinnis it is larger in Nipigon, Superior, and Huron than in 

 Michigan; in kiyi it appears to be smaller in Ontario than in the other lakes; in 

 Jioyi it appears to be smaller in Michigan than in the other lakes. 



Within the species group artedi the races differ in the average value of L/H. 

 Thus, the manitoulinus race has a much larger head, relatively, than the artedi race 

 of Lake Huron, and the albus races of Superior and Erie tend to have a relatively 

 longer head than the artedi races associated with them. 



Fig. It. — Comparison of the heads of Leucichthys zenithicus (A) and L. reighardi (B) of Lake Michigan 



The shape of the head in Coregonus is approximately as in Leucichthys, but the 

 dorsal surface is triangular, due to the shortness of the premaxillaries, and is strikingly 

 convex in the region of the nostrils and occiput. A carina, which is heaviest over its 



Fig. 12. — Comparison of the dorsal view of the head in Prosopiurn (A) and Coregonus (B) 



anterior extent, bisects the triangle. The lateral surfaces also are triangular in 

 shape, obtuse, or acute at the apex, as the angle made by the premaxillaries with the 

 body axis becomes greater than 90°. The ventral surface is like the doral in shape 

 but is only slightly convex from side to side. 



The head in Prosopiurn is quite different from that in the other two genera. 

 The dorsal surface is acutely triangular, owing to the compression of the entire preor- 

 bital region, and is not strongly convex from side to side except in the occipital region. 

 A short but heavy medium keel runs forward from a point approximately above the 

 caudal margin of the eye to its center. A fainter keel originates on each side of it. 



