CAN OWE AN FORMS !^ 



by dorsal and anal elements, D, A" y as in Coelacanthus. 

 The boundary line of the calcified air-bladder, A, is often 

 preserved. 



II. ACTINOPTERYGIANS 



A. Chondrosteans (Ganoids}. Ganoids agree with the 

 Crossopterygians in their exoskeletal characters, although 

 usually lacking in gular plates. The most important 

 differences between these groups have been reduced to 

 those of fin structures ; the Ganoids have no longer the 

 lobate form of the paired fins ; their basal fin supports 

 have become greatly reduced and are usually represented 

 by a single row of a few metamorphosed elements in the 



Fig. 157. The short-nosed gar-pike, Lepidosteus platystomus, Raf. x \. Mis- 

 sissippi basin. (After GOODE in U. S. F. C.) 



most proximal region of the fin. The transitional stages 

 if they exist between the lobate and the monoserial 

 fins have not as yet been demonstrated. 



Fossil Forms 



From the middle of the Palaeozoic period to the end of 

 the Mesozoic there seems to have been a culminating time 

 of forms like the still existing Gar-pike (Fig. 157); their 

 fossils are generally the most numerous, and, on account 

 partly of their strong body armouring of interlocking 

 rhombic plates, the most perfectly preserved of fossil 

 fishes. They usually exhibit the structural characters 



