41 
be applied to the hypobranchial muscles, though from the 
observations at hand we cannot yet conclude that there is 
a complete correspondance. Thus we reach the conclusion 
that in Selachians the number of occipital nerves must 
be approximately indicative of the number 
of post-branchial segments incorporated 
into the skull; approximately, since we have seen 
that the last epi-branchial myotome also may perhaps con- 
tribute to the hypobranchial musculature. Moreover we may 
take into account only those occipital nerves that join the 
cervical plexus and thus pass behind the gill-slits to the 
hypobranchial muscles, for this is not the case with all. 
ot all occipital nerves hypoglossus roots.— In some 
Elasmobranchs, especially in those forms where the number 
of occipital nerves is relatively great (Hexanchus, Heptan- 
chus), the anterior one or two occipital nerves do not 
participate at the formation of the plexus but remain inde- 
pendent and directly run to the epibranchial muscles that 
are supplied by them (cf. fig. 32). This seems to me a circum- 
stance of special interest. FüRBRINGER (l. c. p. 394) con- 
siders itas a secondary emancipation from the plexus. Since, 
however, it is just in primitive forms, as Hexanchus and 
Heptanchus where the nerves designated as v and w are 
independent, that this phenomenon is met with, it appears 
much more probable that we have to do here with a pri- 
mitive feature and that these nerves v and w are to be 
compared with the ventral roots of the anterior, primarily 
epibranchial, myotomes of Petromyzon, being in the latter 
the anterior six post-otic ones. These nerves also supply 
exclusively epibranchial musculature. 
he maximum number of occipital nerves joining the 
cervico-brachial plexus in Selachians is only three, being 
X,y, 2, according to FüRBRINGER. Their branches partly leave 
the plexus again to supply epibranchial musculature, and they 
partly innervate the hypobranchial muscles. By carefully 
splitting the plexus into its components FüRBRINGER could 
demonstrate that it is even subject to serious doubt whether 
the nerve x ever partakes in the innervation of the hypo- 
branchial musculature. In a table given by him (l.c.p.404) 
of the occipital and spinal nerves contributing to the inner- 
vation of the hypobranchial muscles in different Selachians 
we find x with a query, mentioned only for Hexanchus and 
Heptanchus, while at another place in this table only y and z 
