396 
EDWARD PHELPS ALLIS. 
series in the branchial arches of certain fishes (Vetter, 1874), 
and the condition found in the adults of many fishes would 
arise. If such fibres should then separate from the remaining 
fibres of the adductor, a muse ulus praaorbitalis with dorsal 
and ventral muscle bellies, such as is described by Luther in 
certain of the Plagiostomi, would arise. And if the ventral, 
mandibular portion of the muscle became wholly tendinous, 
the condition found in the first and third divisions of the 
levator maxillas superioris of my descriptions of Amia might 
arise. The aponeurosis would naturally tend to be developed 
only where the mouth had a marked posterior extension and 
the opening of the gape was long. According to Luther 
(loc. cit., p. 62) the aponeurosis and the muscle Addy of 
Vetter’s descriptions vary inversely, a marked development of 
the one being associated with a feeble development of the 
other, and he attributes this to the fact that a strongly 
developed Addy would act as a spreader of the jaws, and 
the musculus prasorbitalis, being relieved of that func- 
tion, there would be no call for an aponeurosis. As I 
attribute the development of the aponeurosis to a totally 
different cause it does not seem to me that this applies. 
General Summary. 
The primitive condition of the muscles related to the 
visceral arches of the gnathostorne fishes was, as Vetter 
long ago concluded, a simple constrictor muscle in each arch, 
and associated with this muscle there was a branchial bar 
which lay internal to the muscle. 
From this simple primitive condition two distinctly different 
lines of descent are indicated by later differentiations of the 
muscles, and these differing differentiations are associated 
with, and caused by, two distinctly different forms of branchial 
bar in the branchial arches. One of these two lines of 
descent is represented by the Teleostomi and the other by 
the Plagiostomi, the Holocephali and Dipneusti apparently 
occupying somewhat intermediate positions. 
