112 TRANSACTIONS OF THE AMERICAN PHILOSOPHICAL SOCIETY 



Brough (1931) have added greatly to the detailed morphology both of the braincase and of 

 the surface bones of the skull. In the side view (Fig. \2J) of the skull of Cheirolepis by 

 Watson (1925) we note that the surface bones may readily be grouped on a functional 

 basis as follows: (1) rostro-nasal series; (2) bones of the skull-roof; (3) sclerotic plates; (4) 

 circumorbital series; (5) oromandibular series; (6) preopercular plates; (7) opercular-branch- 

 iostegal series; (8) cleithral series, including the posttemporal and the dermal plates of the 

 pectoral girdle. 



This oldest and most primitive ganoid is a more or less predaceous fish with the large 

 mouth and rather delicate teeth of a gulper. The arrangement of the dermal plates cover- 

 ing the jaws and opercular region depends upon the fact that the primary jaws are serially 

 homologous with the hyoid and branchial arches and that the acts of ingestion, deglutition 

 and respiration are structurally related. It is known from other specimens that the hyo- 

 mandibular, which supports the jaws, is directed backward and this is reflected in the oblique 

 position of the preopercular. 



Some or all of the rostral and postrostral system of palseoniscids apparently gave rise 

 by reduction in number or by coalescence to the "mesethmoid" of Amia and the teleosts. 

 The antorbitals seem to be equivalent both to the nasals and adnasals of Amia and the 

 teleosts (see p. 133 below). 



The eyes are far forward, above the anterior end of the primary upper jaws and not 

 ss large as those of typical fishes. The sclerotic plates are not to be confused with the cir- 

 cumorbital plates of higher fishes. The circumorbital series as a whole is apparently homol- 

 ogous with those of the crossopterygians and oldest tetrapods but the determination of the 

 homologues of the individual bones is uncertain, owing to the variability in the number of 

 the plates in the different genera and families. 



The preopercular is widely expanded on the upper part of the cheek above the maxilla 

 and outside of the adductor mandibulse muscles. In front of it, in Elonichthys and Oxygna- 

 thus, are two small cheek-plates which, as Watson notes, may be the source of the second 

 postorbital row of plates of the Holostei (1928, pp. 58, 59). 



Stensio (1921, p. 139) has proposed the following hypothesis to account for the difficulty 

 in homologizing the crossopterygian, palaeoniscoid and teleost "squamosal" or cheek- 

 plate elements. 



"As we know, there is no cheek-plate in the Actinopterygians that can be directly 

 compared with the squamosal of the Rhipidistids. Watson and Day (1916) have shown, 

 as has been pointed out above, that in primitive forms among the Rhipidistids the squamosal 

 consists of several independent elements, and I now think that this fact can guide us in 

 judging of the conditions in the Actinopterygians. Thus one may venture the supposition 

 that all the homologues of the elements forming the squamosal in the Rhipidistids have 

 never been fused with one another into one bone-plate in the Actinopterygians, but that 

 they were either distributed among the surrounding bones or with partial transformation 

 had been fused into certain larger units which afterwards preserved their mutual indepen- 

 dence or else were finally more and more reduced. In the Palaeoniscids, Platysomids and 

 Catopterids it is conceivable that the original squamosal elements were divided up between 

 the maxillary and the preopercular and that they have even possibly provided material 

 for the so-called postorbitals; in certain Protospondyli, such as the Semionotids and Eugna- 

 thids, they all probably provided material for the majority of the so-called postorbitals 



