A CIPENSEROIDEI 



[17 



Four paired and one median (occipital) supratemporal are present ; 

 also an opercular, a large subopercular, and numerous branchiostegal rays. 

 The exact condition of the rostrum and of the sides of the head is not 

 known, but a suborbital and a small 'jugal' (preopercular ?) are found. 

 There is no median gular ; but about ten branchiostegal rays (Fig. 295). 

 The pterygoids, palatine, dentary, angular, and articular are still present. 

 The jaws are rather small and apparently toothless. The rest of the 

 skeleton closely resembles that of the sturgeons. The Chondrostei are the 

 earliest of this group and occur in the Lias (Egerton, Traquair [451]). 



Fig. 295. 



Chondrosteus drip,,, woides, Egerton. Head and pectoral girdle restored. (After Traquair.) 

 1, frontal ; 2, postfrontal ; 3, parietal ; 4, pterotic ; 5, supratemporal ; 0, post-temporal ; 7, 

 opercular; S, supraclavlcle ; 9, subopercular; 10, clelthrum ; 11, pectoral fin; 12, clavicle; 

 13, branchiostegal; 14, .jugal (preopercular?); 15, angular; 16, eeratohyal ; 17, dentary; 18 r 

 maxilla; 19, suborbital; 20, hyomandibular. 



Chondrosteus, Ag., Lower Lias, and Gyrosteus, Ag., Upper Lias — 

 England. 



Family Polyodontidae. These fish have minute scales on the 

 trunk ; small and separate in Crossopholis, quite vestigial in Polyodon. 

 A Cretaceous genus, Pholidurus, alone retains the ganoine (A. S. Wood- 

 ward). An enormous flattened rostrum without barbels is developed in 

 Polyodon. The roofing bones of the skull are very incomplete and more 

 modified even than in the next family ; few can be recognised (Fig. 294), 

 On the other hand, the mouth and jaws with minute teeth are more 

 normally developed (Bridge [52], Allis [18]). Elongated frontals, 

 parietals, and nasals cover the cranium above ; an interrupted series of 

 median bones runs forwards on the rostrum, but not behind the orbits, 



