266 Journal of Comparative Neurology. 



such cannot be the homologue of the r. mandibularis internus 

 of Anura and fishes. It represents a communis pre-spiracular 

 branch of the faciahs in Selachia. which is shown with special 

 clearness in Squalis acanthias (Green, 'go, Fig. i, p. 416). It 

 is probably the true representative of the chorda tympani in 

 the Ichthyopsida. 



2. The r. mandibularis internus of Anura is a post-spirac- 

 ular nerve and is not represented by an)^ nerve in Urodela, 

 excepting possibly by a small nerve to the r. pre-trematicus IX. 

 There is a difference which appears superficially between 

 the r. alveolaris and the r. mandibularis internus in regard to 

 their manner of origin from the t. hyomandibularis. This 

 difference has been explained by several authors as due to the 

 difference in the topography of the head of the two forms. It 

 may be of more significance than this. Not only does the r. 

 alveolaris arise much nearer than does the r. mandibularis 

 internus to the geniculate ganglion, but there are certain 

 anatomical modifications of the cranium in some species which 

 must be significant. In Proteus and Menobranchus, for 

 example, Druner (591, 607) describes the hyoniandibular 

 trunk as passing out of the cranium in two divisions in such a 

 manner that the the r. jugularis becomes separated from the r. 

 alveolaris by an osseous projection of the cranium which unites 

 by syndesmosis with a similar projection of the suspensorium. 

 Moreover, a still greater modification appears in Siren. In 

 this Urodele Wilder ('91) describes the r. alveolaris as passing 

 out of the cranium in union with the r. palatinus. Such a 

 difference as this from the usual arrangement in Urodela cer- 

 tainly cannot be explained on the basis of topographical modi- 

 fication of the head. On the contrary, it seems to indicate that 

 the r. alveolaris of Urodela occupies a position which is mor- 

 phologically anterior of the r. mandibularis internus. 



Other data, also, lead to the same conclusion. The r. 

 mandibularis internus, in fishes, as a part of the t. hyomandib- 

 ularis, passes to the lower jaw posteriorly of the spiracular 

 cleft. In Anura it holds the same position with reference to 

 the tympanum and Eustachean tube. Now, the Eustachean 



