83 
the myocommata, just like the rudiments of the neural 
arches in the trunk of which they represent the direct 
forward continuation. SEWERTZOFF had numbered the myo- 
tomes in his figure as 6-11, after the system of VAN WYHE 
who counts the “praemandibular segment” as the first and 
the hyoid segment as two segments. 1 have changed this 
nomenclature into 4 — 9, in accordance with the conclusions 
arrived at in the foregoing pages. To each myotome a 
dorsal ganglion and a ventral root corresponds but under 
the vagus ganglion two myotomes are found (4 — 5). This 
is in accordance with HATSCHEK’s and VAN WYHE’s opinion 
that the vagus in Gnathostomes is a bivalent nerve and that 
its primary ganglion has fused with the first spinal ganglion 
behind it (“spinalartiger Vagusanhang”). According to NEAL 
(1898, p. 238), SEWERTZOFF (1899, p,‚ 287) and other investi- 
gators one more spinal ganglion (that of my segment 6) 
afterwards fuses with that of the vagus (vago-accessorius) 
which accordingly in the adult Selachian is trivalent 
(cf. GUTHKE, 1906, and ZIEGLER, 1908), being composed 
of the primary vagus ganglion and two rudimentary spinal 
ganglia following behind it. 
According to HOFFMANN (1894, p. 628) and SEWERTZOFF 
(1899, p. 302) the cranio-vertebral limit in Acanthias lies 
behind my somite 8, in Scyllium and Pristiurus, according 
to VAN WYHE (1882) and others, behind my somite 7. 
The neural arch rudiment between the somites 5 and 6 
corresponds to the first of the four vertebral rudiments 
described by HOFFMANN and has been compared by 
SEWERTZOFF with the occipital arch of Amphibians and 
with the first free arch in Petromyzon. Thus the ventral 
root belonging to somite 5 and to the “spinalartiger Vagus- 
anhang” must be the one that has been observed in Uro- 
delans (cf. p. 49). It must be called x in Amphibians after 
FüRBRINGER’s nomenclature, not z which indicates the last 
head segment of Scyllium. 
À faint neural arch rudiment in front of the somite 5 
(fig. 31) evidently represents a rudimentary prae-occipital 
Arch, as described by Miss PLATT (1898, p. 448) in 
Necturus and by GOODRICH (1911, p. 104) in Siredon. 
As in Amphibians, the two anterior post-otic somites no 
longer develop myotomes, but the second post-otic somite 
Still forms a rudimentary one. From the reniaining occipital 
Somites, however, myotomes are still developed. From these 
