558 ALLIS. [VoL. XII. 
wholly separate muscle, lying immediately above and in close 
contact with the levator, but having no connection whatever 
with it except at its origin. It arises from the entire lateral 
edge of the squamosal back as far as the upper end of the pre- 
operculum, and from the postorbital process behind and above 
the origin of the levator. The surface of origin of the two 
muscles at this place is continuous, but the muscles them- 
selves are, from the beginning, markedly distinct and separate. 
The dilatator runs backward across the upper end of the 
hyomandibular, along its outer surface. It passes through 
the narrow opening between the hyomandibular and the upper 
end of the preoperculum, and is inserted by a tendon, which 
forms on its outer surface, on the inner surface of the oper- 
culum above and in front of the facet forming the articulation 
of that bone with the hyomandibular. 
In specimens of 12 mm. in length and under, the dilatator 
is, at its origin, united to and a part of the levator, the two 
muscles arising, as a single muscle, from the upper edge of 
the cartilaginous postorbital process immediately in front of 
the blind, upper end of the spiracular canal, which, at this age, 
lies entirely outside the cartilage of the cranium. The united 
muscles pass backward, external to the spiracular canal, and 
then separate. The levator is inserted on the inner surface of 
the vertical process of the metapterygoid and on the inner sur- 
face of the metapterygoid membrane behind it. Both mem- 
brane and muscle extend backward beyond the front edge of 
the hyomandibular, lying external to that bone and having no 
apparent connection with it. The muscle here is inserted 
entirely on the membrane, and the membrane shows no trace 
of attachment to the hyomandibular, but vanishes, as the 
muscle does, in the general tissues superficial to it. The outer 
division of the adductor mandibulae, A2, has its origin, at this 
age, from the outer surface of this same membrane, there being 
no attachment whatever to the cranium. The dilatator ex- 
‘tends further backward than either the levator or the adductor, 
and vanishes apparently closely attached to the under surface 
of the dermis. 
