43 
certain influence on the next following spinal nerves, which 
causes the ganglia of the latter to atrophy. In such groups 
where the occipital region of the skull is composed of a 
restricted number of segments, as we shail show below to 
be probable for Amphibians, this sphere of influence may 
extend beyond the cranio-vertebral limit and cause the an- 
terior spinal nerves to lose their ganglia and dorsal roots, 
as is the case in Amphibians. In pentanch Selachians the 
influence of the vagus reaches aiso a little beyond the 
cranio-vertebral limit, but in Acanthias, where the skull con- 
tains one segment more than in Scyllium etc., we find in 
the last segment a well-developed spinal ganglion (cf fig. 31) 
which, however, does not produce a regular dorsal root. 
In hexanch and heptanch Selachians, however, a spinal 
ganglion and a dorsal nerve-root is found over the last 
occipital nerve, fusing with this ventral root in the same 
way as is the case with the spinal nerves of the trunk. 
This indicates that these forms probably have one or two 
segments more incorporated into the skull than Acanthias, 
the influence of the vagus no longer reaching as far as 
the cranio-vertebral limit. 
From these considerations, however, it follows that FüR- 
BRINGER is wrong in designating the occipital nerves of 
Hexanchus and Heptanchus as v, w‚ x, y‚ 2, sin is 
is not the same nerve as z in pentanch forms like Scyllium. 
FüRBRINGER himself (l.c. p. 362) designates the last occi- 
pital myotome in Acanthias as a. Following this nomen- 
clature 1 think it probable that the last head segment in 
Hexanchus and Heptanchus must be called b, or perhaps 
even c. The nomenclature of the occipital nerves in these 
forms must be changed accordingly. Already GEGENBAUR 
(1872, p. 30) had pointed out that the skull in these sharks 
passes more or less gradually into the vertebral column 
and that during life an emancipation of vertebral elements 
from the skull may occur. Thus the backward extension 
of the skull is not constant in different Selachians and the 
shortest skull seems by no means invariably to be found 
with the most primitive forms. f 
It was only after 1 had written down the above conclusion 
resulting from our conception of the hypoglossus-roots that 
| found to my great satisfaction that VAN WYHE (1905, 
P. 320), in a preliminary note on an investigation which 
until now has not been published in full, had reached 
