3i8 



A C1PENSER0IDEI 



The postfrontals and pterotic seem to have fused, and behind theni 

 comes an enormous post- temporal. The bones at the side are still 

 further reduced than in Acipenser. The operculum and suboperculum 

 are vestigial and the interoperculum and branchiostegals have disappeared. 

 The palatopterygoid cartilage has only one ossification in front (palatini I . 

 There are no ribs, and the endoskeleton is very little ossified. The lateral- 

 line canals on the head are to a great extent freed from the large bones 

 and enclosed in special ossicles (Allis [18]). 



Polyodon has a spiracle and a psendobranch, but no hyoidean hemi- 



The spoon-bill sturgeon, Polyodon folium, Lac. Ventral and side view. (After Good e, 



from Dean.) 



branch. The gill-rakers are especially developed. The air-bladder is 

 cellular. 



In the mouth and jaws the Polyodontidae are less, in the rostrum 

 and bones of the head they are more, specialised than the Acipen- 

 seridae. 



Pholidurus, A. S. W. ; Upper Cretaceous, England. Grossopholis, Cope ; 

 Eocene, N. America. Psephurus, Giinther (Fig. 297) ; China. Polyodon, 

 Lac. (Spatularia) ; Mississippi (Fig. 296). 



Fig. 'J'.'T. 

 Psephurus gladius, -Mart. (After Giinther.) 



Family Acipenseridae. The typical arrangement of the cranial 

 roof-bones is much disturbed by the disappearance of some and the sub- 

 division of others. The paired dorsal series can still be recognised 

 (Fig. 298), but they are separated by a number of median osteoscutes, 

 of which a large posterior occipital probably represents the median 

 supratemporal of Chendrosteus. Small scutes cover the very prominent 

 rostrum, below which hangs a transverse series of four sensory barbels 

 (Fig. 300). The mouth becomes peculiarly modified, suctorial, protrusible, 



