80 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



teeth, but occasionally, by overdevelopment of the premaxillary, the maxillary is 

 excluded from the margin of the jaw. 



Instead of the single pterygoid of higher vertebrates there are three bones, an 

 entopterygoid adjoining the palatine, a mesopterygoid (ectopterygoid) which ex- 

 tends back to the quadrate, and a metapterygoid above the quadrate. When the 

 hyomandibular cartilage ossifies it forms a hyomandibular bone from its upper por- 

 tion and a symplectic (an element not known outside the teleostomes), which sup- 

 ports the quadrate. A small bone, the interhyal, intervenes between the hyoman- 

 dibular and the rest of the hyoid. The hyoid copula consists of several elements, 

 the anterior, which supports the tongue being called the entoglossal, the posterior, 

 which connects with the branchial arches, the urohyal. The fifth gill arch consists 

 of a single element on either side, the hypopharyngeal bone, which usually bears 



FIG. 80. Pterygoids, suspensorium and operculum of mackerel (Scomber} after Allis. 



For letters see fig. 68. 



teeth, the two sides being fused in the plectognaths, forming a pharyngeal jaw. 

 The upper elements of the other arches are frequently expanded, bear teeth, and 

 are called epipharyngeal bones. 



DIPNOI. In the three existing genera the skull is comparatively uniform, but 

 the fossils, beginning in the Devonian, have a wide range of structure. In the 

 former the cavity of the chondrocranium extends to the ethmoid region and the nasal 

 capsules have a second opening, corresponding to the inner nares (choanae) inside 

 the oral cavity. The pterygoid is fused with the cranium (autostylic) and there are 

 one (Protopterus} or two (Ceratodus} labial cartilages connected with the nasal 

 capsules. In Ceratodus there are no cranial cartilage bones, but in the other 

 genera a plate composed of fused ex- and supraoccipitals occurs. 



The membrane bones are few, but their homologies are not always certain. 

 The roof is largely formed by an unpaired bone in the position of frontals and 

 parietals, in front of which is a median bone (supraethmoid or fused nasals) above 

 the nasal capsules. In Ceratodus a bone of uncertain homology occurs on either 

 side of the fronto-parietal, but it is lacking in the others, unless it be represented in 

 Protopterus by a pair of bones which abut against the supraethmoid and overlap 



