32 
is therefore probably not homologous with the other oper- 
cular bones. In the Plaice it is L shaped and above over- 
laps a portion of the hyomandibular. Its straight anterior 
margins are closely connected with the hyomandibular, 
symplectic, and quadrate. Opposite the ventral edge of 
the hyomandibular it almost completely covers the inter- 
hyal. Ventrally it overlaps the posterior edge of the 
quadrate. It is the stoutest of the opercular bones—its 
anterior surface especially being strongly calcified. 
Eyeless Side. 
The relations of the bones are precisely the same, but 
the following differences in shape, &c., may be noted :— 
The operculum is slightly smaller and not quite so 
strongly calcified, and the sub-operculum, though smaller, 
is somewhat stiffer than the right. The pre- and inter- 
opercula are both distinctly smaller and less calcified, and 
hence the asymmetry is most marked in the anterior 
opercular bones. 
The bonest which enter more or less into ithe forma- 
tion of the entire skull of the Plaice may now be pro- 
visionally arranged in the following categories according 
to their manner of origin :— 
A.—Bones formed as ossifications within the primi- 
tive cartilaginous brain case of the embryo : —Alisphenoid 
(ALS.), Basioccipital (B.0.), Epiotic (L'p.0.), Exoccipital 
(Ev.0.), Mesethmoid (2.EL., M.E.1), Opisthotic (Op.0.), 
Prefrontal (P.Fr.), Prootic (Pr.O.), Pterotic (Pt.O.—less 
the fused dermal pterotic or squamosal), Sphenotic or 
Postfrontal (Sp.O.—less the fused dermal sphenotic), 
Supraoccipital (S.O.). 
+ The only definite cartilages in the Plaice’s skull are the ethmoid 
cartilage (Hth.), inter-maxillary cartilage (J.M.C.) and Meckel’s cartilage. 
The remaining cartilage is not of an independent character, 
