98 COMPARATIVE ANATOMY 



later unite with the exoccipitals. A new and important modifica- 

 tion as compared with Fishes is the presence of an aperture, the 

 fenestra ovalis, s. vestibuli, on the outer and lower side of each 

 capsule, and corresponding to part of the original space between 

 the capsule and the parachordal cartilage. This fenestra is closed 

 by a cartilaginous plug, the stapedial plate, which is connected with 

 the quadrate and paraquadrate (see p. 82) by ligament, or by a 

 cartilage or bone (columella auris), the two structures probably 

 together corresponding phylogenetically to the upper section of the 

 hyoid arch (hyomandibular), though this homology can no longer 

 be traced ontogenetically. The olfactory capsules are well 

 developed and arise in part independently and partly in connection 

 with the converging trabeculse. In Necturus and Proteus they are 

 delicate and fenestrated, and united with the cranium by connec- 

 tive tissues only. 



The snout is limited anteriorly by the toothed premaxillae, 

 which usually more or less completely enclose a cavity (inter- 

 maxillary or internasal sinus) containing a gland. Each external 

 nostril is bounded by the nasal process of the premaxilla, the 

 nasal, and the toothed maxilla, and a small investing bone, the 

 septomaxillary, is also present between the maxilla and nasal in 

 relation with the nostril. The premaxillse and maxillfp form the 

 upper boundary of the gape. Between the nasal and maxilla is a 

 prefrontal, and medially to this a frontal, followed behind by a 

 parietal, which partly covers the auditory capsules. 



Forming the greater part of the skeletal roof of the oral cavity 

 and strengthening the skull-base is a large and broad parasphenoid 

 (Fig. 72), which, as in Fishes, is sometimes provided with teeth. 

 It extends forwards from the occipital region to the olfactory cap- 

 sules, closing over the basicranial fontanelle, and ventral to it is 

 the paired and toothed vomero-palatine bar, the two elements 

 comprising each of which become fused in adult Urodeles, but vary 

 much in form and arrangement. The vomerine part of this bar 

 is situated beneath the olfactory capsule and is in contact with the 

 premaxilla and maxilla, thus helping to strengthen this region, at 

 the posterior part of which is the internal nostril, situated much 

 more posteriorly than in Dipnoans. Internally to the suspen- 

 sorium is a pterygoid bone, a process of which extends forward 

 towards the maxilla. 



The suspensorium is much more simple than that of Fishes 

 (Figs. 72 and 73). It consists of the palatoquadrate only, with 

 a quadrate ossification, and has usually four typical processes con- 

 necting it with surrounding parts (pedicle or basal process, otic, 

 ascending, and pterygoid processes). The quadrate l becomes fused 

 secondarily with the skull, and on its outer surface is an investing 



1 In Tylototriton verrucosus the quadrate sends forwards a process which 

 connects it with the maxilla, and thus forms a lower zygomatic arcli or infra- 

 temporal arcade. 



