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liead-segment a more or less developed spinal ganglion, which however 

 no longer pioduces a dorsal roof. In He.cnnchus and Heptanchus, 

 however, the last occipital nerve is provided with a well-developed 

 dorsal- root with a ganglion, so here the influence of the vagus no 

 longer reaches quite to the end of the skull, which in these forms 

 passes more or less gradually into the vertebral column. This holds 

 still more for the Chondrostei, where in several species a considerable 

 number of dorsal and ventral roots leave the skull (Furbkingkr, 

 1897, p. 450) and where even the whole brachial plexus can have 

 been incorporated into it (ibid., p. 457). For the supposition that in 

 Hexanchus and Heptanchus the longest skull among Selachians is 

 found, we maj also point out the well developed primarily epibranchial 

 nerves (v, iv, x of Fürbringer). In pentanch sharks the hindmost of these 

 roots, as a consequence of the decreasing number of gill-slits, pass 

 into post-branchial or hypoglossus-roots. The same holds for the 

 development of the epibranchial musculalure. 



On the other hand in rays the cranium appears to be shorter, the 

 number of occipital hypoglossus-roots is mostly 0, never more than 1 

 (Fürbringer, 1897, p. 404), and the influence of the vagus reaches 

 part of the way behind the cranio-vertebral limit, both the anterior 

 spinal ganglia being absent (Fürbringer, p. 392). There is no epibranchial 

 musculature or occipital nerves supplying it. All this points much 

 more to a phylogenetic decrease than to an increase of the length 

 of the skull in the Selachians. 



So in Elasmobranchs we have on the whole a partly intracianial, 

 partly post-cranial hypoglossus the anterior roots of which as a rule 

 are lying within the sphere of influence of the vagus and hence 

 lose their dorsal glanglia. Gegenbaur (1871, p. 521) called (he 

 occipital nerves "ventral vagus roots", which is right, if we 

 consider the vagus as a partially polymeric nerve and moreover 

 bear in mind that in Selachians we could better speak of a vago- 

 accessorius. With the already bivalent vagus-ganglion the rudimen- 

 tary ganglion of the 6'^ segment still fuses (Neal, 1898, p. 238), 

 so that the vagus ganglion is now trivalent, fused from one normal 

 and two rudimentary ganglia. 



In Amniotes the number of occipital myotomes observed during 

 ontogeny nearly corresponds to that of Selachians. The number of 

 intracranial (occipital) hypoglossus roofs is very generally stated to 

 be 3, thus one more than in Acanthias, but the number of gill-slits 

 (5) being one less than in Acanthias, we must conclude that the cranio- 

 vertebral limit almost corresponds to that of the latter form and that the 

 skull here loo contains some eight segments. Both in Selachians and 



