small ganoid scales. The eye was large and far forward on 

 the head. The operculum was large but separate septa ex- 

 tended out from each of the hyoid and branchial arches. 

 The eye was encircled by five plates; the operculum was 

 supported only by branchiostegal rays. 



The most primitive ray-finned fishes are represented by 

 such genera as Cheirolepis and Perleidus (Figure 2-16). These 

 had strongly developed dermal bones covering the head, a 

 rhomboid cover of thick scales with a distinct glassy enamel 

 layer, a single dorsal fin, a heteroceral tail, an anal fin, and 

 two pairs of lateral fins. The membranous part of the fins 

 was supported by jointed fin rays. There was a small fleshy 

 lobe at the base of the fin. 



Acenlrophorus (Figure 2-17) is the earliest known holostean 

 fish, and in it the holostean features of cheek and jaw were 

 fully developed. In the somewhat later occurring Walsonulus, 

 or Parasemionotus, there was an interesting combination of 

 features of the holostean and palaeoniscoid. 



The crossopterygian fishes (Figure 2- 1 8) are choanate fishes. 

 They are best represented by Osteolepis and Eusthenopteron, 

 which show many similarities to the assumed ancestor of 



the Amphibia. These fishes had thick dermal bones cover- 

 ing the head and operculum. The body was covered by 

 rhomboid or cycloid scales of the cosmoid type (based on 

 their histology — see Chapter 8). Each had two dorsal fins, a 

 heterocercal or diphycercal (double) tail, an anal fin, and 

 two pairs of lateral fins with fleshy lobes. From these fleshy 

 lobes with their "fringe" of web the term Crossopterygii is 

 derived. In these fishes the fleshy lobes are not nearly as 

 large as those of the dipnoans, nor more extensive than 

 those of Latimena. Dipterus was an early Devonian member 

 of the Dipnoi. The body was covered by cycloid scales of the 

 cosmoid type. There was the same pattern of fins as that 

 seen in the crossopterygian fishes. 



The earliest of the amphibians belonged to the ichthyo- 

 stegid group from the uppermost Devonian of Greenland 

 (Figure 2-19). This kind of animal is imagined as looking 

 much like a salamander. Some of the later labyrinthodonts 

 were very large with proportionally larger heads and shorter 

 tails. They are assumed to be modified in structure as com- 

 pared with the primitive fish-like form. 



opercular fold 



ICHTHYOSTEGA 

 Figure 2-19. The Devonian amphibian kthyostego as it may have looked. 



FOSSIL VERTEBRATES • 33 



