No. 446.] ORAL BREATHING VALVES OF TELEOSTS. 159 



The reason for this structure is easily seen when the floor of the 

 oral cavity is examined. 



This floor is raised in the center and hollowed at each side in 

 such manner as to divide the outgoing Stream into two, and the 

 corresponding part of the roof is so shaped and hollowed as to 

 deflect the stream somewhat, so that notwithstanding the supe- 

 rior position of the mouth, the upper valve is necessary. 



In Echensis iiaucmtis, where the mouth is truly superior, there 

 is a small maxillary valve, whose greatest width is on the parts 

 each side of the meson about half way to the ends. This also 

 can be accounted for by the hollowing out of the roof of the 

 oral cavity. 



In HeviirJiampJius nnifasciatus the mouth appears suj)erioi-. 

 but it is more truly horizontal, the appearance being produced 

 by the great prolongation of the mandible. This species has 

 the maxillary valve but little smaller than the mandibular. 



Pomolobus psendoharengus affords a very characteristic example 

 of the development of the mandibular valve. The mouth being 

 very oblique and narrow, the 

 mandibular valve is very deep 

 and "baggy." (Fig. 8.) 



In Chilomycterns schoepfi 

 the valves are crescentic, the 

 mandibular valve being four 

 times the width of the maxil- "' 

 lary valve. The valves are 



covered with coarse papillae. They entirely close the mouth 

 when the jaws are at full stretch, which has been noted in no 

 other species. 



A good example of deep valves is found in Oncorhyncliu:i 

 nerka, a salmon. In this fish the jaws are hooked, long and 

 somewhat compressed. If the valves were narrow, there would 

 be a long, free margin. Consequently the valves would have to 

 be very heavy to resist the outward pressure from within against 

 the unsupported margins. As it is, the free edges lie far back in 

 the mouth, their length is much reduced and the surface of the 

 valve, which is far better able to stand the strain, increased and 

 hung in a better position for resistance between the bones of the 

 jaws. (Fig. 9.) 



