VISCERAL ARCHES OF THE GNATHOSTOME FISHES. 879 
Huge (1897), that when the intermandibularis is innervated 
wholly by the nervus facialis, a muscle of facialis origin has 
simply crowded out and replaced one of trigeminus origin, 
but in a later work (1913, p. 46) he concluded that the 
trigeminus muscle here persisted, but had secondarily 
acquired innervation by the nervus facialis. Because of the 
wide distribution of the innervation of certain fibres of the 
intermandibularis by the nervus trigeminus, he considers 
this to be an archaic feature in fishes. 
In Chlainydoselachus, I find the musculi interliyoideus and 
intermandibularis forming a single continuous muscle-sheet 
which extends transversely from one side of the head to 
the other, without the intervention of a median aponeurosis. 
The posterior quarter, approximately, of this muscle-sheet 
is inserted, on either side, on the corresponding ceratohyal, 
while the anterior half is inserted wholly on the mandibula. 
Between these two parts of the muscle-sheet, and lying im- 
mediately anterior to a tendinous band which extends from 
the musculus adductor mandibulm to the musculus inter- 
hyoideus (see Luther, 1909, fig. 1), I find, in all my speci- 
mens, the fibres of the remaining quarter of the sheet 
separated, for a short distance along each lateral edge, into 
deeper and superficial layers, the deeper (dorsal) fibres 
being inserted on the ceratohyal and the superficial (ventral) 
ones on the mandibula. The deeper layer lies external to 
the proximal (anterior) portion of the ventral end of the 
constrictor of the first branchial arch, but in large part 
separated from it by the hyal branchial rays and the hyo- 
branchial gill pouch. The constrictor of the first branchial 
arch similarly overlaps and lies external to the constrictor of 
the second branchial arch. This overlapping of these 
muscles is well shown in Vetter’s figure of Heptanchus 
(1874, PI. 15, fig. 7), where the proximal fibres of the con- 
strictores superficiales of the first and second branchial 
arches, are shown lying directly internal to the musculus 
interhyoideus. 
There are accordingly here, in Chlamydoselachus and 
