GREGORY: FISH SKULLS 439 



through the intervention of the small rod-like interhyal, which fits in a socket furnished by 

 the symplectic and the preopercular. But when the upper pharyngeal teeth become very 

 powerful, as in the scarids (Fig. 134), they are furnished with a pedestal by the parasphenoid. 

 In the Cyprinidae, where the enlarged lower pharyngeals work against a horny pad, the 

 latter is supported by an oval pedestal supplied by the basioccipital. 



Responses to Cranial Diverticula of the Swim-bladder 



The neurocranium is also moulded by certain prolongations of the swim-bladder, which 

 put forth finger-like tips that, as it were, burrow their way into various places on the side 

 of the cranial vault. This happens quite independently in the clupeoids (p. 147), mormy- 

 roids (p. 169) and, in a quite different way, in the Ostariophysi (p. 184). 



Moulding Influence of the Eyes 



THc sizes and positions of the paired sense organs naturally influence profoundly the 

 characters of the neurocranium. For example, when the eyes are greatly enlarged, as in 

 Opisthoproctus (Fig. 43) and Periophthalmus (Fig. 229C), the interorbital bridge is con- 

 stricted and the ethmo-vomer block is thrust forward. At the same time the circumorbitals 

 tend to extend far backward over the cheeks as in the Semionotidae (Fig. 22). It would 

 seem also that the lunate arrangement of the preopercular, opercular and cleithral series 

 is to a considerable extent conditioned both by the large size of the eyes and by the small- 

 ness of the mouth in larval stages (see p. 422) and that the basic teleost pattern may have 

 been derived from a larval stage of an adult fish with small eyes and large predaceous jaws 

 of the palaeoniscoid type (see p. 112). Conversely, the reduction of the eyes, as in mormy- 

 rids (p. 170), naturally permits the interorbital bridge to widen and the parethmoid process 

 to become reduced. 



The size and position of the eyes naturally determine very largely the position and 

 special characters of the interorbital bridge, or at least of the rear surface of the ethmo- 

 vomer block, as well as the characters of the postorbital process of the sphenotic and of the 

 entire postorbital arch or front wall of the cranial vault. When the large eyes are moved to 

 the top of the skull the interorbital bridge may become very narrow, as in Astroscopus 

 (Fig. 248), while certain of their muscles, forced to withdraw from the myodome on the 

 base of the skull, fill a great space behind the orbits and become transformed into electric 

 batteries; these in turn push back the front wall of the cranial vault. In Salmo one pair of 

 the eye-muscles (the superior oblique) extending forward invade the ethmo-vomer block 

 and doubtless influence the form and position of the mesethmoid septum in ways not yet 

 sufliciently understood (Goodrich, 1909, p. 325). In the flat-fishes, where the eyes are 

 twisted into new positions in accordance with the inclination of the body to one side or the 

 other, the interorbital bridge is pushed in front of the migrating eye and a secondary brace is 

 developed from the lateral ethmoid and the sphenotic (Figs. 224, 225). 



In the acanthurids and still more in the balistids (Figs. 161-164) the eyes have moved 

 upward and backward to such an extent that they actually override the hyomandibulars, 

 pushing the suborbitals into contact with them, merging the interorbital bridge with the 

 roof of the cranial vault and causing the pterotic process to point downward rather than 

 backward. The opposite specialization is seen in Cyclothone (Fig. 54), in which the very 

 small eyes are pushed far forward to the front end of the interorbital bridge. In the gulpers 



