GREGORY: FISH SKULLS 



121 



from an actinopteran starting-point, such peculiar resemblances to the sharks as the union 

 of the opposite palatoquadrates beneath the braincase may well be regarded as examples of 

 convergence. 



Acipen ser 



scalebonp f/adj 



Chondrosteus 



Fig. 19. Skulls of (A) modern sturgeon, and (B) its Jurassic ancestor, Chondrosteus . Both after \. S. Woodward. 



The degenerative specialization of the sturgeon seems to me equally clear and convinc- 

 ing. The excessive concrescence of vertebral elements with the occiput, the presence of 

 a valvula cerebelli, the extreme reduction of the mouth and jaws and their suctorial char- 

 acter, the development of rostral barbels, the loss of ganoine on the exoskeleton and the 

 development of ridged bony plates on the skin, are a few of the many aberrant specializations 

 of the sturgeons away from a typical actinopteran starting-point. The resemblances of the 

 rostrum, mouth and lips of the sturgeon to those of Acanthias, noted by Sewertzoff (1928), 

 are so superficial that they indicate with high probability that such resemblances between 

 the modern chondrosteans and the elasmobranchs are largely convergent and I would 



