658 ALLIS. [Vov. XII. 
applied externally to the lower posterior portion of the latter 
and to the base of its opercular process. The thin hind edge 
of the deeper portion of this part of the preoperculum overlies, 
and corresponds with, the thin hind edge of the hyomandibular 
between the opercular process and the lower corner of that 
bone. A groove on the inner surface of the preoperculum, 
corresponding with the posterior or hyoidean portion of the 
facial depression on the outer surface of the hyomandibular, 
marks where the hyoideus facialis passes downward and 
backward between the two bones. 
The symplectic lies, as van Wijhe has stated, nearly at right 
angles to the hyomandibular, and the two bones are separated 
and connected by a triangular interspace of cartilage. The 
anterior corner of this piece of cartilage is overlapped and 
covered externally by the posterior, cartilaginous corner of the 
palato-quadrate arch. The rest of the piece, excepting only a 
very small portion, is covered externally by the preoperculum, 
and the hind edge of the piece is turned outward, upward and 
forward against the inclined hinder and inner surface of the pre- 
operculum, and firmly attached to that surface by tissue. On 
this turned-up portion lies the oblong articular facet for the 
epihyal. The long axis of the facet lies, as does the epihyal, in 
a plane perpendicular to the lower edge and to the outer sur- 
face of the hyomandibular, but the axis of the facet is inclined 
at a considerable angle to the plane of the hyomandibular, 
instead of being at right angles to it, as might have been ex- 
pected. The facet therefore lies on the inner surface of the 
cartilaginous interspace, instead of on its hind edge, and the 
relatively long front edge of the epihyal is inclined to the axis 
of that piece, instead of being at right angles to it. 
d. Operculum and Branchiostegal Rays. 
The three opercular bones, the operculum (OP), suboperculum 
(SOP), and interoperculum (JOP), fit with their anterior edges 
under and against the outer, posterior edge of the preoperculum 
(POP), separated from it on the outer surface of the undissected 
head by a fold or crease in the dermis, and separated also from 
the first branchiostegal ray by a similar fold or crease. The 

