176 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



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. Yentrally 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 ihe 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 

 (Al.S.), Basioccipital (B.O.), Epiotic (Ep.O.), Exoccipital 

 (Ex.O.), Mesethmoid (M.E., M.E. 1 ), Opisthotic (Op.O.), 

 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.). 



f The only definite cartilages in the Plaice's skull are the ethmoid 

 cartilage (Eth.), inter-maxillary cartilage (I.M.C.) and Meckel's cartilage. 

 The remaining cartilage is not of an independent character. 



