GREGORY: FISH SKULLS 



177 



strong, so that in spite of its weak premaxillae the fish ought to be able to pluck up small 

 creatures living on the bottom, such food habits being also indicated by the downwardly- 

 turned mouth, down-bent rostrum and sensory barbel. 



It is obvious then that some of the peculiar characters of the skull of this fish are con- 

 nected with the presence of a blunt projecting rostrum and with the development of a 

 small and peculiarly specialized downwardly-directed mouth. Thus if we were to endeavor 

 to make a pictorial restoration of the skull of the remote ancestor of this fish prior to these 



epiot 

 ... pto 



PXO 



GONORHYNCHUS 



Fig. 66. Gonorhynchus greyi. Side view. After Ridewood. 



specializations, we should have to shorten greatly the rostrum, free the opposite frontals 

 on the mid-line, greatly enlarge the jaws and bring the mouth forward to its primitive 

 anterior position. This would give us a generalized isospondylous skull, not unlike that 

 of Leptolepis. 



But there are other specializations in this* skull whose functional interpretation is not 

 difficult. On the dorsal surface of the second basibranchial bone Ridewood figures (p. 

 366) a circular patch of "about twenty strong blunt teeth, which engage with the teeth 

 on the entopterygoids, and with the latter constitute the entire dentition of the animal." 

 Thus Gonorhynchus like various other isospondyls chose, as it were, to develop teeth on the 

 floor and roof of the mouth and to sacrifice the teeth on the jaws, and in so doing it evolved 

 rounded peg-like teeth. But in this case these were located exclusively on the second 

 basibranchial and opposite entopterygoids instead of being spread over several bones on 

 the floor and roof of the mouth. The upward thrust of these basibranchial teeth on the 

 roof of the mouth, under the pull of the branchial levator muscles, would tend to disrupt 

 the ordinary edge-to-edge contact between the thin entopterygoid and the thin edge of 

 the hyomandibular. Hence it is not surprising to find that in this fish the metapterygoid 

 is represented by a thin rod of bone, running from the lower end of the hyomandibular 

 upward and forward to the entopterygoid. Thus this rod together with its ligaments 

 would be in an excellent position to check the upward thrust of the basibranchial teeth, 

 while the rod-like symplectic would perform a like office for the quadrate, steadying it 

 against the wrenching movements caused by the pull of the adductor mandibulse muscles 

 on the relatively strong mandible. The marked forward displacement of the mandible 



