THE AMERICAN NATURALIST. [Vol. XXXVIII. 



In the Catostomidse, or suckers, the case is very different. 

 Here the mouth is inferior, (Fig. 7, c), and the snout depressed. 

 In this family the maxillary valve is well developed and very 

 deep. There are two reasons for this. First, the mouth of the 

 sucker is not wide and must be opened farther; second, the 

 lower valve is entirely absent. The reason for this is obvious. 

 The depressed snout decurves the regurgitated stream, which 

 strikes the upper valve only, hence a lower valve is not neces- 

 sary, the edge of the upper valve catching against the lower jaw. 



The position of the maxillary valve in these fishes is nearly hori- 

 zontal, this being a better position to catch the returning stream. 

 In the case of the minnows whose mandibular valve is replaced 

 by papillae, and in the case of some other fishes with maxillary 

 valves much larger than the mandibular valves, the mouth is 

 apparently horizontal or even slightly oblique. In all such cases, 

 however, either the snout is depressed or the premaxillary is 

 protractile and is held protracted when the fish is breathing 

 quietly, thus depressing the snout and deflecting the greater 

 part of the water against the maxillary valve. 



As may be inferred from the foregoing, the fishes with true 

 oblique mouths possess large mandibular and small maxillary 

 valves. The latter are never completely absent but may be very 

 small. The case of Astroscopus y-grceciim, the stargazer," 

 might be supposed to form an exception to this rule, as the 

 mouth is really superior. From analogy it might be supposed 

 that the maxillary valve would be lacking. Such, however, is 

 not the case. There is a well developed maxillary valve (Fig. 

 3), like two crescentic valves joined at the meson by their ends. 



