906 THE AMERICAN NATURALIST. | [Vor. XXXIX. 
These muscles (Figs. 5, 7, 8, 11, /ntör) are fewer in number 
and lie between the demibranchs of the four complete gills. 
Each is extremely thin and consists of a sheet of fibers, anterior 
to the cartilaginous gill rays, and extending between the gills 

Fic. 8.— Lateral view of. Acanthias, showing the interb hiales (u£br). T, trapezius. 

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from top to bottom of the septum. A tough white fascia 
extends from the dorsal longitudinal muscles to the extra- 
branchial cartilages, holding the latter in place below the apo- 
neurosis between the dorsal constrictors, and connecting them 
with the interarcual muscles to be described later. The most 
dorsal of the interbranchial muscle fibers arise from the fascia 
just described, while the rest have their origin in the aponeurosis 
between the dorsal constrictors. The more median of the fibers 
curve outward and downward toward the median line, to become 
inserted on the anterior margin of the epibranchial cartilage 
(Fig. 7). The more lateral fibers are larger and follow a broad 
curve from the dorsal to the ventral side of the gill septum, and 
thus pass into the ventral fibers about to be described, just as 
the dorsal and ventral constrictors merge in part into each other. 
The ventral fibers arise from the extrabranchials as well as 
