42 
are mentioned for these same forms. In all other species 
only 2, 1 or O occipital roots of the hypoglossus are presen. 
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 Pristiurus 
comprises 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 the 5tb and 6t® post-otic somite (cf. 
Plate l). Now according to NEAL, as we have seen, the hypo-_ 
branchial musculature in Acanthias originates from the 4t*- 8th 
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 micht expect three, according to HOFFMANN two, occipital 
hypoglossus-roots. In Acanthias there are only two (FüRBRIN- 
GER, lc.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 FüRBRINGER 
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 FÜRBRINGER's statement needs 
confirmation. VAN WYHE's (1882, p 16) observations do 
not give evidence on this point. 
Variation in lengih 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 O 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 4 
