i08 THE ANCESTRY OF VERTEBRATES 



are mentioned for these same forms. In all other species 

 only 2, 1 or occipital roots of the hypoglossus are present. 

 Thus the number of occipital hypoglossus roots in Selachians, 

 as far as stated with certainty, never exceeds 2. 



We have seen already that according to the researches of 

 Van Wyhe and others the skull in Scyllium and Pristiiirus 

 compiises one post-branchial segment. In Acanthias, accor- 

 ding to HOFFMANN (1894, p. 638) and SewertzOFF (1899, 

 p. 302), there is one segment more incorporated into the 

 head, the number of post-branchial somites accordingly 

 being two, corresponding to i\ic 5*^^ and 6*"^ post-otic somite(cf. 

 Folate 1). Now according to Neal, as we have seen, the hypo- 

 branchial musculature in Acanthias originates from the 4*'^- S**' 

 post-otic somite, of which consequently the three anterior 

 belong to the skull, while in HOFFMANN'S opinion the 4'" 

 post-otic somite does not participate. Thus according to NEAL 

 we might expect three, according to HOFFMANN two, occipital 

 hypoglossus-rocts. In Acanthias there are only two (FiiRBRlN- 

 GER, l.c.p. 362) in the adult form, which is in fair accordance 

 with our conclusions. In Scyllium where, as we have seen, there 

 is only one postbranchial occipital segment FuRBRlNGER 

 found two occipital nerves joining the brachio-cervical 

 plexus, and in young stages three, of which, however, the 

 anterior one only supplies epibranchial muscles. Evidently 

 we must assume either that in Scyllium the last epibranchial 

 somite contributes to the formation of the hypobranchial 

 musculature, to account for the presence of two occipital 

 hypoglossus-roots, or that FuRBRINGER's statement needs 

 confirmation. VAN WVHE's (1882, p 16) observations do 

 not give evidence on this point. 



Variation in length of the Selachian skull. — If our conclu- 

 sions until now are right, we may, as stated above, derive the 

 approximate number of post-branchial segments incorpo- 

 rated into the skull in Selachians from the number of occipital 

 hypoglossus-roots, which may vary from to 2. The less 

 this number is, the greater is as a rule the number of post- 

 cranial, spinal, hypoglossus-roots (rays!). Now in heptanch 

 and hexanch Selachians the number of occipital hypoglossus- 

 roots is never less than .2, and since the number of gill- 

 slits is 1 or 2 more than in pentanch sharks we can only 

 conclude that the skull contains more segments than in the 

 latter. This conclusion is supported by another circumstance. 

 The strong development of the vagus evidently exerts a 



