98 PJSCES 



to the condition found in the lower Pentadactyle vertebrates 

 (Amphibia and most Reptilia). In these the palato-quadrate arch 

 is not only continuous in front with the ethmoid region of the 

 cranium, but also behind with the auditory i-egion. The quadrate 

 is forked proximally ; the lower limb, or pedicle of the suspensorium, 

 is usually fused with, or firmly fixed to, the anterior outer region of 

 the ear capsule ; the upper limb, or otic process, is fused to this 

 capsule above. In the canal so formed, between the limbs of the 

 quadrate and the capsule, the hyomandibular branch of the facial 

 nerve passes backwards to its peripheral destination, crossing over 

 the spiracular gill-cleft (or Eustachian tube and tympanic cavity, 

 Fig. 59, B). The postspiracular hyomandibular cartilage takes no 

 share in the support of the upper jaw. This is the typical 

 autostylic attachment, which among the Pisces is found in the 

 Dipnoi only. 



In the amphistylic Notidanidae, the superior otic process (Fig. 

 59, A) would appear to be represented by the large upgrowth of the 

 quadrate region of the upper jaw, which articulates with the post- 

 orbital process of the auditory capsule, and passes outside and 

 above the hyomandibular nerve. In the anterior palato-basal 

 process, Avhich articulates more ventrally with the cranium behind 

 the optic foramen, Huxley saw the representative of the ' pedicle 

 of the suspensorium.' The relation this process bears to the 

 nerves, however, shows that this view is untenable. Possibly the 

 pedicle is not represented at all in Elasmobranchs, and the anterior 

 process probably represents merely the ethmoid process, found 

 articulating in front of the orbit in most fish. It has shifted 

 back in the Notidanidae, which are provided with an exceptionally 

 wide gape. 



Gegenbaur looked upon the articulation of the otic process as 

 representing the primitive attachment of the arch [153]. Huxle} r , 

 on the contrary, considered it to be secondary [230]. It would 

 certainly seem that the mandibular arch must have been originally 

 attached more ventrally, below the nerve exits (by the palato-basal 

 process) like the other visceral arches. Nevertheless, the otic 

 articulation appears to have been established very early, since 

 there is reason to believe that it existed not only in the Jurassic 

 Cestraciontidae (Hybodus, p. 144), but also in the Cladoselachii 

 (p. 185), Acanthodii (p. 189), and Pleuracanthodii (p. 180). 



The notochord secretes an outer thin elastica externa and an 

 inner fibrous sheath, often very thick (Hasse [201], Klaatsch [265], 

 Schauinsland [384]). Outside there is the mesoblastic skeletogenous 

 layer (Fig. 60). Neural arches develop in this layer, and also 

 haemal arches, which in the trunk are represented by lateral basal 

 processes (parapophyses). In some fish, Polypteridae and the 



