MOTOR NUCLEI IN PHYLOGENY ies) 
two nuclei obtains in the selachians charted in figure 17 than in 
many other sharks hitherto examined (vide 66, 72, etc.) and since 
it is known that ganoids have lost certain occipital nerves which 
are characteristically present on sharks, but little if any reli- 
ance can be placed upon the results of such a direct comparison 
as that suggested above. 
A comparison of this region among ganoids themselves (fig. 
25) brings out the fact that the extent of the vagus column 
caudad of the exit level of the first spino-occipital rootlet, as 
well as the extent of overlap of the caudal vagus column and 
spino-occipital nucleus, is greatest in Acipenser and least in 
Polyodon, while in these respects Amia and Lepidosteus occupy 
an intermediate position in the series. An examination of the 
relative development of the musculature to which the vagus is 
distributed shows that these central relations closely agree 
with the peripheral conditions obtaining in these forms (ef. 
discussion of spino-occipital complex). 
Thus, though the trapezius be absent in Acipenser, the levatores 
arcuum branchialium externi which are closely related to this 
muscle ontogenetically, are more numerous in this form than in 
Amja (MecMurrich, |. ¢.) and they are chiefly innervated from 
the caudal vagus column. Further, in Amia (Allis) and in 
Lepidosteus (Edgeworth) in both of which the trapezius is a 
very small muscle, the only coraco-branchialis present in these 
forms (pharyngo-clavicularis externus) receives its innervation 
from the caudal part of the vagus (cf. supra, innervation of coraco- 
branchiales in Acipenser). Finally in Polyodon, where a re- 
duction of the caudal part of the vagus nucleus appears to be 
most evident among ganoids, the pharyngo-clavicularis is inner- 
vated not by the vagus but by the first spinal (Danforth), the 
trapezius though present is small, and the levators of the bran- 
chial arches are not so extensive or so numerous as in Acipenser. 
°If the so-called pharyngo-claviculares muscles of Polyodon are homolo- 
gous with the coraco-branchiales muscles of Acipenser, as their respective 
nerve supply would suggest, then in this respect both these forms resemble 
selachians, while in Amia and Lepidosteus, on the other hand, the pharyngo- 
claviculares muscles receive their innervation in a manner similar to that 
obtaining among most teleosts. 
THE JOURNAL OF COMPARATIVE NEUROLOGY, VOL. 27; No. 4 
