GANOIDEI. 95 



PALiEICHTHYES. 



Order VII. GANOIDEI. 



The fishes belonging to this Order have the skeleton either 

 cartilaginous or ossified ; the body is provided with median and 

 paired fins, the hinder pair abdominal ; the gills are free,, rarely 

 partially attached to the walls of the gill-cavity ; one external gill- 

 opening only on each side, and a gill-cover ; the air-bladder with 

 a pneumatic duct. The ova are small, impregnated after exclusion. 

 The embryo or the young sometimes with external gills. 



To this Order belong the majority of the fossil fish-remains of 

 Palaeozoic and Mesozoic age, whilst it is very scantily represented in 

 the recent fauna, and evidently verging towards total extinction. 

 Small as is the number of the surviving forms, they represent not 

 less than five Suborders : — 1. Amioidei. 2. Polypteroidei. 

 3. Lepidosteoidei. 4. Dipnoi. 5. Chondrostei. 



Amioidei. — The sole living representative of this Suborder, the [Case 28.] 

 Mud-fish of North America (Amia calva), differs from the following 

 Ganoids in being covered with cycloid scales, and approaches in its 

 general appearance and many points of its internal structure the 

 Teleostean type very closely indeed. The skeleton is entirely 

 ossified. This fish is not uncommon in many of the fresh waters 

 of the United States. 



Polypteroidei (Case 28). — They resemble the Lepidosteoidei m 

 the form and arrangement of the scales, but the structure of their 



Polypterus. (From Tropical Africa.) 



dorsal fin is quite unique ; it consists of a series of dorsal spines, to 

 each of which an articulated finlet is attached. Polypterus (fig. 86) 



H 



