400 
EDWARD PHELPS ALLIS. 
large adductor mandibulae actually found in the adult. 
From those portions of the primarily continuous constrictor 
that lay dorsal and ventral, respectively, to the palatoquadrate 
and mandibula, the musculi levator maxillae superioris and 
intermandibularis were developed. The musculus levator 
anguli oris was probably derived from the anterior edge of 
the united levator and adductor muscles before they became 
separated from each other. The musculus intermandibularis 
underwent relative reduction, and, in the adults of recent 
fishes, is largely crowded out and replaced by those super- 
ficial fibres of the hyal constrictor that have secondarily 
acquired insertion on the mandibula. The relations of the 
nervus hyoideus facialis to the muscle fibres thus inserted on 
the mandibula is against the view that those fibres that are of 
mandibular origin have lost their primitive innervation by 
the nervus trigeminus and secondarily acquired innervation 
by the nervus facialis. 
The posteriorly directed dorsal and ventral ends of the 
hyal and branchial constrictores of the Selachii always over- 
lap, to a greater or less extent, the next posterior constrictor. 
Where the ends of the constrictores are strongly inclined 
posteriorly, they may overlap two or more posterior con- 
strictores, the fibres of the muscles then, crossing the 
extrabranchials of those arches and there tending to become 
tendinous exactly as they do where they cross the extra- 
branchials of their own arches; a series of tendinous 
aponeuroses thus being formed in each constrictor. The 
overlapping muscles then fuse more or less completely with 
each other, and, as the linear aponeuroses related to each 
extrabranchial are superimposed and transverse to the muscle 
fibres, the continuous muscle-sheet formed by the fusion of the 
several constrictores is cut up into what have heretofore 
been considered to be separate segments, one related to 
each branchial arch and developed entirely from the myotome 
of that arch. These segments are, however, each formed by 
muscle fibres derived from two or more consecutive constric- 
tors, and hence from a similar number of consecutive myotonies- 
