514 DAVIDSON BLACK 
spond in structural detail, as Johnston (49) long ago pointed out 
to be the case in Acipenser. The acusticum and lobus lneae 
lateralis in Polyodon are relatively even larger than in Acipenser 
and together with the cerebellum form by far the greater bulk 
of the rhombencephalon. It is possible that the poorly circum- 
scribed but recognizable nucleus paramedianus in Polyodon and 
in other ganoids has been differentiated under the influence of 
these structures and does not represent the more or less degen- 
erated remains of a once more highly developed nucleus. By 
this it is implied that the acusticum, lobus lineae lateralis and 
cerebellum together in ganoids bear the same causal relation- 
ship to the differentiation of the paramedian area as does the 
cerebellum to this area within the groups in which the latter 
organ has independently gained a high state of relative speciali- 
zation, viz., sharks, birds, and mammals. 
Motor vagus nucleus (Nu. mot. N. X.). The trapezius muscle 
in ganoids has become much reduced as compared to selachians, 
a condition which is probably due in a large measure to the 
development of the operculum and consequent general modifi- 
eation of the musculature in the branchial region. 
In Acipenser the trapezius is represented in the 11 mm. em- 
bryo (Edgeworth) but, according to Vetter, disappears in the 
adult. In Amia, Lepidosteus, and Polypterus, Edgeworth de- 
scribes the development of the trapezius. In the first named 
form this muscle, which is described by Allis (1. ¢., p. 669) as 
the ‘fifth externus’ levator of the branchial arches, is inner- 
vated by a branch of the vagus. In Polyodon also the trapezius is 
present though not a large muscle, and is innervated by a long 
branch of the vagus (Danforth). 
It is to be expected from what has been said in the case of 
selachians that a reduction of the trapezius musculature such as 
obtains in ganoids would lead to some curtailment in the caudal 
extent of the vagus nucleus. Judging from a comparison of the 
extent of the overlap of the caudal viscero-motor column and 
spino-occipital nucleus in the selachians and ganoids charted 
(figs. 17 and 25), some shortening of the vagus nucleus in the 
latter forms is apparent. However, as a greater overlap of these 
