SKELETON 89 



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, dermal ethmoid 

 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 the fronto-parietals. The otic capsule 

 and quadrate are covered by a squamosal, and the roof of the mouth is formed 

 by a large parasphenoid, in front of which are a pair of palatines. In advance of 

 these last are a pair of large teeth resting directly on cartilage, their bases repre- 

 senting the greatly reduced vomers. The lower jaw has three bones on either 

 side, a small dentary, a larger angulare, and an enormous splenial, which alone 

 bears teeth. 



In Ceratodus there is a hyomandibular fused to the cranium behind the exit of 

 the seventh nerve, but elsewhere there is only the hyoid. The operculum has 

 one or two elements (operculare and interoperculum) the free edges of which bear 

 cartilaginous rays, and the gill arches are five in Ceratodus, six in the other genera. 

 A peculiar feature of Protopterus and Lepidosiren is the so-called cranial rib, a 

 slender cartilage bone articulated with the chondrocranium below the occipital 

 plate, and extending backward and downward across the shoulder girdle (fig. 

 90, cr). 



In those extinct Dipnoi which are united with the recent genera to form the 

 order Sirenoidea, the skull is much as in the existing forms, except for the more 

 numerous bones. In the Arthrodira (formerly called placoderms) the cranium 

 is hinged to a large plate which covers the anterior part of the trunk, and the 

 skull is roofed with a few large plates, some of which may be homologized with 

 those of the sirenoids, the others not being readily compared with the bones of 

 other vertebrates. The suggestion has been made that the problematic fossil 

 PalcEospondylus resembles, in its skull, the larvae of the dipnoans, the adults of 

 which were common in the same seas. 



AMPHIBIA. — Several points distinguish the amphibian from other skulls. 

 The chondrocranium (fig. 91) is platybasic (p. 68); except for a small synotic 

 tectum frequently present, it is not roofed by cartilage; the otic capsule has a 

 fenestra vestibuli occupied by the stapes, a development connected with the 

 power of hearing (p. 81); there are two occipital condyles; and the quadrate is 

 immovably united to the cranium by two processes, an otic process, joining the 

 otic capsule, and an 'ascending process' which reaches the upper margin of the 

 trabecula (o/>), and which, in many reptiles, often ossifies as the epipterygoid 

 bone. 



The cartilage cranial bones are few. Usually ordy exoccipitals are developed 

 in the hinder region, while the rule is a single petrosal (prootic), but occasionally 



