85 
dorsal roots, is incorporated into the skull (FüRBRINGER, 
1897, p. 457). 
Thus among the so-called occipital nerves we must 
distinguish primarily epibranchial, supplying epibranchial 
musculature, from secondarily epibranchial or hypoglossus 
roots, supplying hypobranchial musculature. From the 
number of the latter may be determined the approximate 
number of post-branchial segments into which the skull 
reaches. Adding to this number that of the epibranchial 
pl. cerv. 
Fig. 32. Plexus cervico-brachialis of Heptanchus, 
after FüRBRINGER'’s statements, : 
l primarily epibranchial ventral roots 
2 cervical plexus or hypoglossus. 
plexus brachialis. 
segments we get the total length of the skull. This proves 
to be subject to variation among Selachians, the number of 
epibranchial (depending on the number of gill-slits) as well as 
that of post-branchial segments may vary. Thus FüRBRINGER 
was wrong in assuming that the skull in all Selachians 
has equal length and that consequently the last segment 
and its ventral nerve may always be indicated with the same 
letter (z). If in Scyllium we call it z, we ought to call it 
a in Acanthias and b or c in Hexanchus and Heptanchus. 
While z and a in the latter form are primarily epibranchial 
nerves, they have become hypoglossus-roots in forms like 
Scyllium where the number of gill-slits has decreased. In 
