FOSSIL FISHES. 



23 



like those of Dipterus, but the scales are thin and the tail probably 

 diphycercal. Teeth indistinguishable in character from the modern 

 Ceratodus are abundant in the Trias and Khsetic. The other genera 

 are Palcedaplius, Heliodus, Phaneropleuron (Wall-case 5 a), from 

 the Old Eed Sandstone, and Uronemus, Ganopristodus, from the 

 Carboniferous rocks. 



[Ganoidei, proper.*] 



The fishes of this order are, in most instance?, covered with 

 osseous ganoid scales or scutes — the ventral fins are always 

 abdominal. 



The skeleton in some is notochordal, or cartilaginous, in others 

 partially or completely ossified. 



Fig. M.—Pohjpterus bichir, living in the Nile, Gambia, &c. a, Pectoral fin ; b, ventral ; 



c, anal fin. 



Only one external gill-opening exists on each side, protected by 

 a gill-cover. 



Among living Ganoids there are four well-marked tyne3 : — 



Sub-order IV. — Crossopterygld^:. 



The Crossopterygidce, which are represented by JPolypterus (Fig. 

 34) and Calamoichtliys, are confined to the rivers of tropical Africa 

 (especially those of the West coast) and the Upper Nile. 



Sub-order V. — Acipenseboidet. 



The Acipenseroidei are represented by Acipenserfi the Stur- 

 geons, of which there are twenty species which frequent European, 

 Asiatic, and American rivers. The skeleton in these fishes is noto- 

 chordal ; the skull is cartilaginous, with dermal ossifications and 

 scutes. 



* The three previously-noticed sub-orders — Acanthodini, Placodermata, and 

 Dipnoi — are doubtfully classed under the Ganoidei : from the remarks of Prof. 

 Huxley, Dr. R, H. Traquair, and others, it seems quite probable that these ancient 

 types of 6shes may hereafter form more than one distinct order. 



+ To this family also belong Scaphirhynchus, Polyodon, and Pseplmrus, found in 

 the rivers of Asia and North America. 



