GREGORY: FISH SKULLS 



119 



The sturgeon (Fig. 19) has specialized in the opposite direction from that of the 

 primitive chondrosteans, as it has acquired an excessively small suctorial mouth which is 

 withdrawn far behind the projecting rostrum. The enormous hyomandibular is directed 



Polyodorv folium. 



R s;)atKu(a 



Fig. 17. SkuW oi Polyodon folium. Mainly after Traquair. 



backward but the greatly enlarged symplectic reaches downward and forward. These ele- 

 ments are covered with broad and thick muscles, which alternately dilate and retract the 

 hyomandibular, thus pumping water into the oropharynx. The eye remains relatively 

 far forward above the anterior part of the upper jaw. The so-called opercular is free of the 

 hyomandibular and probably corresponds to an enlarged subopercular. The whole snout 

 and fore part of the braincase is warped downward above the capacious orobranchial cavity 

 in order to bring the snout down parallel to the ground. The downward displacement of 

 the rostrum probably increases the adverse leverages of the fore part of the skull upon the 

 occipito-nuchal joint. Hence the extensive fusion of cervical segments with the occiput 

 may be a means of compensating for this weakness. The rostral barbels are specialized 

 tactile organs, more or less similar in function to those of siluroids, gonorhynchids, mullids, 

 sciaenids, gadids, etc. 



The neurocranium of the sturgeon and spoonbill are largely cartilaginous but with more 

 or less extensive centers of ossification. It has been assumed by Watson and Stensio that 

 this partly cartilaginous condition is due to retrogressive development (perhaps to the 

 retention of early larval conditions in the adult). Sewertzoff, however, as a result of his 

 embryological investigations (1928) challenges this view and concludes that the recent 

 chondrosteans are much more nearly related to the elasmobranchs than was formerly 

 suspected and that in many respects they are more primitive than the Palaeozoic palasonis- 

 cids. He holds among other things that the numerous ossicles in the snout of the sturgeons 

 are more primitive than the few rostral elements of the palaeoniscids. 



