362 
EDWAKD PHELPS ALLIS. 
Fibres of the ramus post-trematicus internus of the next 
posterior arch are said to also enter the muscle, but Danforth 
could not determine whether they were motor or sensory nerves. 
In Acipenser, Vetter (1878) found a small adductor in each 
of the first three branchial arches. 
Adductores arcuum branchialium are accordingly described 
only in the Elasmobranchii and Ganoidei, and in these two 
groups of fishes there is marked difference, not only in the 
position of these muscles, but also in their manner of inner- 
vation. The muscles in these two groups of fishes cannot, 
then, be homologous if the innervation of muscles, and the 
relations of nerves to skeletal structures, are as constant as I 
consider them to be. That the nerves that innervate the 
muscles in Amia and Polyodon have cut through the 
related epibranchials, from their anterior to their pos- 
terior surfaces, the perforation of the cartilage in the 
Plagiostomi representing an intermediate stage in this 
process, I, on principle, greatly doubt, and, furthermore, 
DohriPs observations offer a different and more probable 
explanation of the conditions in the latter fishes. According 
to that author (1884, p. Ill), a concentration of mesoderm 
cells takes place, at a certain stage in embryos of these 
fishes, posterior to the proximal edge of the related myotome, 
and soon afterwards a second concentration of similar cells 
takes place anterior to the myotome. These two groups of 
cells are said to represent the beginnings of the chondrifica- 
tion of the branchial bar of the arch, but it is not said how or 
when the two groups fuse. That part of the myotome of the 
arch that lies between the two groups of cells is said to 
later differentiate as the adductor of the arch, and it would 
seem as if the nerve that innervates the muscle so differen- 
tiated would of necessity lie between the two groups of ce Is, 
and hence later perforate the branchial bar, and this seems 
to find confirmation in conditions that I find in my 42 cm. 
specimen of Scyllium. In this fish each musculus adductor lias 
its insertion, at either end, in a pit in the related epibranchial 
or ceratobranchial, and in each of the ceratobranchials this pit 
