120 Cc. H. DANFORTH 
Action. The combined muscle acts as an opponent to the 
protractor and also raises the hyomandibular and dependent 
structures. The posterior fibers tend to constrict the opercular 
aperture. Obviously ‘levator’ would be quite as appropriate a 
term for this muscle, but since both Vetter and Gegenbaur desig- 
nate the same muscle in Acipenser as ‘retractor,’ I have retained 
their nomenclature. 
IV. MUSCLES OF THE BRANCHIAL ARCHES 
Within the membranous septum between each pair of demi- 
branchs there is developed a lamina of striated muscle, M. inter- 
branchialis, the fibers of which are grouped in irregular bundles 
which extend chiefly from the cartilage on the (morphologically) 
posterior edge of the groove for the. efferent branchial artery 
diagonally across the septum to its anterior lateral margin. Very 
few fibers take the other diagonal course so as to cross these. 
The innervation was not definitely determined but obviously the 
supply comes from the neighboring fused pre- and post-trematic 
rami which are the only nerve fibers in the vicinity. It is difficult 
to state their function definitely. Pulling on the septum, they 
probably tense the filaments which are borne on it and throw 
them into a position favorable for the circulation of water among 
them. 
Besides this musculature, which would perhaps be more appro- 
priately described as the intrinsic musculature of the organs of 
respiration, there are for each branchial arch three additional 
muscles, one at the median end of the dorsal half of the arch, 
one between the two moieties and one at the ventro-median end. 
Posteriorly there is a single transverse muscle above and one 
below. These muscles will now be described in the order named. 
Muscles relating the branchial arches to the shoulder girdle will 
be discussed in another section. 
MM. levatores arcuum branchialium: figure 4, m.lev. 
The four levator muscles of the gills arise as a continuous 
sheet from a broad line on the back of the chondrocranium 
beneath the area of origin for the retractor hyomandibularis. 
The anterior fibers are the most ventral and have their origin 
