324 



BULLETIN OF THE BUREAU OF FISHERIES 



slightly farther craniad, and extends to the nares. The lateral surfaces are roughly 

 ovoid in shape. They are nearly flat to a line on a level with the superior edge of the 

 maxillaries and from thence converge sharply in a downward direction, the more 

 sharply as the snout is approached. The ventral surface also is acutely triangular 

 in form but is strongly convex from side to side. 



It appears from Table 8 that the head is smallest in proportion to the total 

 body length in this genus. There seems to be variation in this character; the race 

 of Michigan seems to have a proportionally smaller head than those of Superior 

 and Huron. 



Brain Box 



An examination of the bones of the skull shows the prefrontal bone to extend 

 almost completely over the orbit in Leucichthys and the carina of the frontals to 

 extend to the frontal-parietal suture. In Prosopium the prefrontal is but little 

 developed and does not extend much beyond the anterior edge of the pupil; the 

 cranial carina does not extend to the frontal-parietal suture. In Coregonus the 

 development of these structures is about as in Leucichthys. 



Premaxillaries 



The shape and position of the premaxillaries serve to separate the three generic 

 groups and to aid in the separation of reighardi and zenithicus from the other species 

 of Leucichthys. In Leucichthys the premaxillaries are longer than wide and make 

 an angle not in excess of 90° with the horizontal axis of the body behind them. 

 This angle usually is between 60° and 75° for typical reighardi and zenithicus and 

 45° and 60° for the others, including the dymondi form of reighardi. In Coregonus 

 and Prosopium they are wider than long and the angle is always in excess of 90°. 

 The angle may vary within the species; the premaxillaries are less perpendicular 

 in the dymondi race of reighardi than in the typical one. 



Snout 



The shape of the snout depends, of course, upon its length and on the position of 

 the premaxillaries. It is more blunt or more pointed, according as the premax- 

 illaries are more vertical or more horizontal. The relative length of the snout, as 

 compared with the length of the head, is variable and is not distinctive for any 

 species. It is longest, on the average, in zenithicus. 



The usual length may vary within a species. Johannse has a longer snout in 

 Huron than in Michigan; zenithicus has a somewhat longer snout in Nipigon than 

 in Superior and a somewhat shorter one in Michigan and possibly Huron; the 

 reighardi of Nipigon and of northern Lake Michigan has a somewhat longer snout 

 than the typical form, and the Superior form has a somewhat shorter one; the 

 cyanopterus subspecies of nigripinnis has a relatively longer snout than the other 

 races; Tciyi in Superior has a somewhat shorter snout than in the other lakes. 



Maxillary 



The shape and size of the maxillary and the supplementary maxillary (jugal) 

 are more or less distinct for the three groups. In Leucichthys both of these bones 

 are elongated more than in the other two groups. While in Leucichthys the max- 



