462 WILLIAM K. GREGORY 



(8) gular plates slowly give way to branchiostegals as the head 

 becomes narrower; (9) the splenials disappear from the mandible ; 

 (10) the hinder expansion of the maxillary gives rise to a separate 

 supram axillary; (11) the preoperculum withdraws from the 

 cheek and comes into closer relationship with the pterygopalatine 

 series, assisting in the support of the quadrate (note 1, p. 457) ; 

 (12) the chondrocranium is now much reduced, and replaced by 

 cartilage bones. 



Cohort Ganoidei (/. Muller, Jordan, and Evermann) 

 Superorder Acipenseroidei Traquair. 

 (Palte XXIX) 

 Notochord persistent, with neural and haemal arches. Teeth 

 small or wanting. Infraclavicles present. Paired fins actinop- 

 terous with a row of baseosts. A single dorsal and anal fin, 

 with dermal rays more numerous than their supports; caudal 

 fin (at least) with fulcra. Caudal fin heterocercal, the upper 

 lobe usually scaly. Chondrocranium apparently but little ossi- 

 fied, the cranial bones mainly dermal. 



Order Heterocerci Zittel 



Notochord persistent, but arches, spinous processes, and fin 

 supports more or less ossified ; opercular apparatus well developed ; 

 branchiostegal rays numerous. Unpaired, and usually also 

 paired fins fringed with fulcra. Scales rhombic or rhomboidal, 

 rarely cycloidal. 



Palseoniscidas Devonian to Upper Jurassic 



Platysomidae Carboniferous and Permian 



Dictyopygidae (Catopteridae) Incertae Sedis Trias 



Order Chondrostei /. Muller 1846. 



Endoskeleton chiefly cartilaginous. Opercular apparatus im- 

 perfectly developed, the branchiostegal rays usually absent. 

 Trunk almost or completely naked or with rows of bony plates. 



Chondrosteidce Lias (Lower Jura) 



Acipenseridae Tertiary and Recent 



Polyodontidae Cretaceous ( ?) Eocene to Recent 



Belonorhynchidas Incertas Sedis Trias and Lias (Lower Jura) 



