36 
they have disappeated. According to DOHRN (1885, p. 
466) and HOFFMANN (1898, p. 265) this little group of 
muscles arises from the occipital somites. They are sup- 
plied by tiny branches from the cervical plexus, as we 
may call part of the cervico-brachial plexus which ín other 
groups of Vertebrates is separate from the brachial plexus. 
The cervical plexus in Selachians is composed of a 
varying number of spinal nerves, from four to twelve, the 
least numbers being found in the sharks, the highest in 
the Rajidae. This can be determined by carefully splitting 
the cervico-brachial plexus into its components. Evidently 
there is a certain relation between the total number of 
roots of the cervico-brachial plexus and the depth to which 
secondarily the series of gill-slits has extended backwards 
into the trunk region, as indicated by the distance of the 
shoulder-girdle from the head. The number of occipital 
nerves in different Elasmobranchs may vary trom |, or 
even 0, to 5, the highest number being met with in the 
most primitive forms (Notidanidae), the lowest in the 
more cifferentiated, such as the rays. They are designated by 
FüRBRINGER with the final letters of the alphabet: v, w, 
Xx, Y, Z, of which then the most anterior ones show a ten- 
dency to disappear. The spinal nerves following behind 
them and forming together with them the cervico-brachial 
plexus are numbered: 1, 2, 3 etc., or, as far asin Teleos- 
teans and Amniotes they are assumed to have been incor- 
porated into the skull (FüRBRINGER’s occipito-spinal nerves, 
the hypoglossus of Amniotes): a, b, c‚, etc. s 
The circumstance that the occipital nerves, though: passing 
through the cranium, yet in Selachians join the cervico- 
brachial plexus, which takes a wide curve round behind 
the last gill-slit to the hypobranchial musculature, indeed 
seems to plead strongly for the post-branchial origin of 
the corresponding occipital somites. For if the occipital 
nerves did ab origine belong to the branchial region, we 
might expect them to pass between the gill-slits and not 
behind them to the ventral side. The same holds good for the 
ventral muscle-buds from the occipital myotomes which 
grow out equally round behind the last gill-slit to participate 
at the formation of the hypobranchial or, casu qü0, 
the tongue musculature, as has been shown e. g. by VAN 
WYHE (1882) for Selachians, by VAN BEMMELEN (1889) 
for Reptiles and by FRORIEP (1885) for Mammals. 
