150 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



in position, slightly in front of the pectoral, and imme- 

 diately in front of the aims. As far as we have seen its 

 fin rays never vary in number, and we have always found 

 six on both sides. This is very striking when we consider 

 the variability in the number of fin rays in all the other 

 fins. 



The opercular fold or gill cover of the Plaice is large, 

 and the branchial cavity opens behind by a wide aperture. 

 This aperture is bounded on the inner side by the clavicle 

 and on the outer side by the loose branchiostegal mem- 

 brane supported by the branchiostegal ra}^s. On lifting 

 the opercular fold the gill-like pseudobranch is easily 

 seen in a slight recess on the inner side of the operculum 

 immediately over the dorsal extremities of the gill arches. 

 Yentrally the opercular folds are separated by a conical 

 fleshy mass containing the " inter-clavicle,'' and known as 

 the isthmus. 



The lateral line of systematists commences on the 

 tail, and courses straight forward at about the middle of 

 the side of the body for the greater part of its length. It 

 curves slightly upwards over the pectoral fin, and there- 

 after becomes buried in the bones of the head. Further 

 portions, however, of the lateral line system are visible on 

 the surface, notably the right infraorbital canal under the 

 right eye, and a portion of the supratemporal canal under 

 the dorsal fin. 



The scales are mostly cycloid, but according to 

 Cunningham (pp. cit.) the so-called " ciliated " or 

 " spinulated scale " is found only in mature males, and 

 may form a conspicuous local peculiarity. The scales of 

 this character that we have seen had three or four blunt 

 processes on their posterior border. 



.Regarding the apertures, the mouth is terminal and 

 markedly asymmetrical. If the mouth of a plaice be 



