SKULL 95 



branchiostegal rays are developed in the ventral parts of the 

 opercular fold or branchiostegal membrane (Fig. 69). 



Anteriorly, the opercular apparatus lies against a bony chain 

 consisting of three pieces the hyomandibular, symplectic, and 

 quadrate which serves as a suspensorial apparatus for the lower 

 jaw (Fig. 69). The latter consists of Meckel's cartilage and of 

 several bony elements, the largest of which is the toothed dentary : 

 the others are the articular, angular, and coronary. The last two, 

 however, may be wanting. The articular is developed in the 

 articular portion of Meckel's cartilage, which latter is ensheathed 

 by the dentary and angular. 



The hyoid arch is usually followed by four branchial arches and 

 a rudimentary fifth which forms the " inferior pharyngeal bone." 

 The dorsal segments of these arches become fused together to 

 form the " superior pharyngeal bone," which, like the inferior 

 pharyngeal, usually bears teeth. 



The skull of Dipnoans is in a sense intermediate between 

 that of Chimaeroids and Teleostomes on the one hand, and that of 

 Amphibians (more especially Urodeles) on the other. In various 

 respects, however, it presents special characters, such as the marked 

 metameric segmentation of the occipital region and the relations 

 of certain of the investing bones. 



The chondrocranium is retained almost entirely in the most 

 primitive existing representative of this group Ceratodus, and to 

 a large extent in the other two genera : the only perichondral 

 bones being a pair of exoccipitals (Fig. 71). The occipital region 

 is firmly connected with the vertebral axis, and the two or 

 three anterior vertebral elements which are united with the skull 

 may possess more or less distinct neural arches and spines (e.g. 

 Protopterus) : the vagus nerve passes through a space between 

 the auditory capsule and first neural arch. 1 A large "cranial 

 rib" articulates with the hinder part of the skull on either 

 side, in a position corresponding to the third occipital neural 

 arch. 



The cranial cavity extends forwards between the orbits to the 

 ethmoidal region (platybasic type), and its front wall (lamina 

 cribrosa) is largely cartilaginous. The cartilaginous nasal capsules 

 are lattice-like, and as in all Vertebrates higher in the scale, each 

 nasal cavity communicates with the mouth by internal nostrils : 

 the external nostrils are covered by the upper ftp. 2 



The ethmo-nasal region is covered by a median dermal supra- 

 ethmoid, postero-laterally to which is a supraorbital bony lamella 

 (" dermal lateral ethmoid "), and articulating with it posteriorly 

 in the median line in Ceratodus is another unpaired bony lamella 



1 In the embryo of Ceratodus it has been shown that there are five mvotomes 

 anterior to this point. 



2 There are two so-called "labial cartilages/' one of which arises from the 

 trabecular region, passing behind and to the outer side of the external nostril, and 

 the other probably belongs to the nasal skeleton. 



