GREGORY: FISH SKULLS 



117 



cids and sturgeons (in the broad sense, including the acipenserids, polyodontids and chon- 

 drosteids), but apparently mostly to the latter; that . . . they really seem to have evolved 

 from the same ancestral form among the primitive actinopterygians as the sturgeons (in 

 the broad sense); that this common ancestral form in its turn has not been a palaeoniscid 



poros 



Pla^somus parvulus 



Fig. 15. Skulls of deep-bodied derivatives of the Pala'oniscidte. 



A. Chcirodus granulosus. After Traquair. 



B. Platysomus parvulus. After Watson, but names of elements changed in some instances to conform to system herein adopted. 



but must in certain respects . . . have been more primitive. In reality it must have been 

 closely related to some primitive, hitherto unknown type of actinopterygians from which 

 the higher ganoids and teleosts also originated." Stensio also concludes that the saurich- 

 thyids, like the sturgeons, palseoniscids, coelancanthids, dipnoans and arthrodires, form a 

 degenerative series. By this he means especially that in such scries the adult endocranium 

 is better ossified, less cartilaginous, in the earlier than in tiie later members of the series. 



Chondrostei (Spoonbills and Stlrgeons) 



According to the views of Traquair (1877, p. 39) and most other palsoichthyologists 

 the existing Polyodon (Fig. 17) represents a specialized and in some respects degraded de- 

 rivative of the primitive chondrostean stock. It perhaps owes its survival to the great 

 development of the tactile snout. The small eye remains above the front end of the 

 upper jaws. The primary jaws are very large, the long hyomandibular being directed 



