370 
EDWAKD PHELPS ALLIS. 
that the ventral end of the constrictor of the hyal arch fuses 
with a muscle developed from a corresponding portion of the 
mandibular myotome. 
Edgeworth says (1911, p. 206) that in 14 mm. embryos of 
Scyllium, the ventral end of the hyal myotome becomes con* 
tinuous with the lateral edge (morphologically the dorsal end) 
of the future interhyoideus, this latter muscle being said to 
be developed, as will be later explained, from the walls of the 
ccelomic cavity and not from the hyal myotome. In 16 mm. 
embryos the myotome is said to be partly continuous with the 
interhyoideus and partly inserted on the lateral surface of the 
hyal bar to form a levator hyomandibularis (levator hyoidei, 
Edgeworth). In later stages the myotome becomes separated 
from the interhyoideus, and the lateral edge (dorsal end) of 
the latter muscle is inserted on the ceratohyal. A backward 
extension of the myotome and the interhyoideus then takes 
place, and a continuous dorso-ventral muscle-sheet is thus 
formed, which lies posterior to the hyal bar, and is said to be 
the muscle C 3 vd of Huge’s (1897) descriptions of the adult. 
This dorso-ventral sheet is accordingly said to be formed, in 
its dorsal portion, by fibres derived from the hyal myotome, 
and in its ventral portion by fibres derived from the ccelomic 
wall. The constrictor superficialis of the hyal arch is 
accordingly not the strict serial homologue of the constric- 
tores of the branchial arches, although this is not so stated 
by Edgeworth. It is said that the primary form of the 
interhyoideus, developed from the coelomic wall, would 
appear to have been a transverse band connecting the two 
hyal bars. 
In the adult Selachii the muscles of this arch have been 
described by Vetter, Tiesing, Ruge, and Marion, Ruge’s 
descriptions being particularly complete. In this arch no 
musculi interbranchialis, arcualis, or interarcualis are differen- 
tiated ; but, according to Dolirn, the two latter muscles may 
be represented by ligaments. A musculus interbasalis 
(Interarcualis dorsalis I, Vetter) may be found extending 
from this arch to the first branchial arch (Allis, 1915), but 
