VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 395 
action as a protractor of the palatoquadrate and also as a 
spreader (Spreizer) of the articular ends of the jaws. I 
consider the aponeurosis to have been developed wholly 
independently of either of these two functions. In my 
opinion, a small adductor muscle had already been differenti- 
ated in this arch before the remainder of the constrictor 
began to slip over onto the anterior surface of the carti- 
laginous bar of the arch. There was, at this period, quite 
certainly not sufficient space between the relatively close 
fitting integument of the arch and the cartilaginous bar to 
permit this large and long constrictor muscle to immediately 
assume the position that the adductor actually has in the 
-adults of living fishes, and the small adductor already 
occupied the angle between the two elements of the carti- 
laginous bar. Certain of the fibres of the constrictor 
accordingly quite certainly acquired attachment on the 
internal surface of the dermis at the angle of the gape 
These fibres would immediately act as an adductor when the 
mouth was widely open, but when the mouth was closed, or 
even nearly closed, they would act primarily as a protractor 
anguli oris, and secondarily as an abductor of the arch ; for, 
being attached to the dermis at the angle of the gape, and 
the dermis being fixed, any contraction of the* muscle would 
necessarily tend to open the mouth. This would evidently 
be of advantage to the fish, for, in the early stages of the 
development of the mouth, there was probably no other 
abductor mechanism, the ventral longitudinal muscles not yet 
having been developed. The fibres so inserted, increasing 
in number and importance, would, as the adductor muscle 
developed and a cheek was formed, pinch off the subdermal 
tissues to which they were attached and an aponeurosis such 
as is actual^ found in Chlamydoselachus, and will be fully 
described in my later work, would almost inevitably arise. 
If these fibres did not become attached to the dermis they 
would, in certain cases, become tendinous as they passed 
across the angle of the gape, as they do when passing 
over the extrabranchials and the middle rays of the branchial 
