450 Julia B. Platt 
(v. WiJHE '82, NEAL '97, Horrmann ’97) nor does this first postotie 
segment give rise to muscle fibres in any other Vertebrate above 
the Selachii of which I know. According to SEWERTZOFF himself, 
in Acipenser, »Die Reihe der Myotome endigt vorn gerade gegen- 
über dem N. vagus. Nach vorn vom N. vagus habe ich beim Sterlet 
keine Myotome gesehen« (l. e. page 36). 
SEWERTZOFF finds that in Siredon, the first ventral spinal nerve 
lies opposite the third myotome, i. e. the third postotic somite, 
since the first postotie somite develops muscle fibres. In Necturus, 
the first ventral nerve lies in the fourth postotic segment. In 
Siredon, the occipital arch falls between the second and third postotic 
segments. In Necturus, between the third and fourth. In Siredon, 
the position and fate of the rudimentary muscle developed in M,, 
corresponds to the position and fate of the rudimentary muscle 
developed in the ventral part of the second postotic somite in Nec- 
turus. In short, we find that there appears to be one more segment 
in the head of Necturus than in the head of Siredon, and that 
this segment is the first postotic, in which no muscle fibres 
develop in Necturus. 
Supposing, however, that we assume that the same fusion be- 
tween the dorsal parts of the second somite and third takes place 
in Siredon as in Necturus, making one apparent segment of the 
dorsal parts of two, and that in Siredon, as in Necturus, the 
ventral part of the second somite becomes separate from the dorsal 
part, and supposing further that we assume that SEWERTZOFF mistook 
this ventral part of somite 2 for the rudimentary somite 1, then 
Siredon would correspond with Necturus, and it would not appear 
that a rudimentary muscle arises exceptionally in the first postotic 
somite, where in no Vertebrate above the Cyclostomata such a 
muscle is known to appear. 
If this assumption be true, as it is certainly plausible, then one 
segment more than SEWERTZOFF thought enters the head of Siredon, 
and there are, as in Necturus, three instead of two occipital 
segments. 
SEWERTZOFF describes the posterior vagus root, and homologizes 
it with the N. aecessorius (l. e. page 64), but failing to distinguish 
the two vagus segments opposite which the two vagus roots respectively 
lie, SEWERTZOFF seems not to have recognized their segmental value. 
As above mentioned, the ventral part of the second postotie 
segment gives rise to muscle fibres which are found for some time 
