VIII. 2 PALAEONISCIDS 231 



around the eyes. The outer margin of the upper jaw is covered by 

 premaxillae and maxillae, which are the main tooth-bearing bones. 

 Behind the orbital series of bones the cheek is very variable. Sometimes 

 there is a large bone identifiable as a pre-opercular, with a series of 

 opercular bones behind it. The lower portion of the throat was covered 

 by a series of gular plates. The spiracle in these early forms opened 

 above the opercular bones. The pectoral girdle was attached to the 

 back of the skull by a supracleithrum, below which a cleithrum and 

 clavicle made a series of dermal bones behind the gills, covering the 

 cartilaginous girdle. The roof of the mouth contained a median para- 

 sphenoid, with paired prevomers in front of it, and a series of 

 pterygoid bones occupied the space between it and the edge of the 

 jaws, the palatine, ectopterygoid, pterygoid, and sometimes others. 

 Finally the lower jaw, besides the main dentary carrying the teeth, 

 shows many small bones such as the pre-articular and coronoid on 

 the inner surface; splenial, angular, and surangular on the outside. 



It will be clear that this skull of *Cheirolepis may be closely com- 

 pared with the skull of a crossopterygian or a modern teleostean. 

 The general plan is related to that of the lateral line organs arranged 

 along occipital, supratemporal, and infra-orbital lines. The numerous 

 small bones are evidently similar in the different groups, though it is 

 not easy to assign a suitable name to every one of the more numerous 

 bones of the earlier forms. 



These palaeoniscids from the Middle Devonian were rather rare 

 freshwater fishes ; they had sharp teeth and probably lived on inverte- 

 brates. We have no information about their internal anatomy, but it 

 seems not unlikely that the air-bladder possessed a wide opening to 

 the pharynx (as it still does in Polypterns, descended from this stock) 

 and that they breathed air, as did other Devonian fishes. However, 

 they did not have internal nostrils, which are found in the old crosso- 

 pterygians. 



During the Carboniferous and Permian the palaeoniscids were 

 numerous, mostly as small, sharp-toothed fishes. Several distinct lines 

 became laterally flattened and acquired an outwardly symmetrical tail 

 and blunt crushing teeth (Fig. 147). These characteristics probably 

 indicate a habit of feeding in calm waters, perhaps mainly on corals, 

 and they have appeared several times in the actinopterygian stock 

 (p. 241). Palaeoniscids of this type were formerly placed together in 

 a family Platysomidae, but it is now considered probable that the 

 type arose independently several times; thus * Amphicentrum is found 

 in the Carboniferous, *PIatyso?nus and *Dorypterus in the Permian, 



