146 TRANSACTIONS LIVERPOOL P.TOLOGTOAL SOCIETY. 



Introduction. 



The subject of the present Memoir belongs to the old 

 group of fishes known as the " Teleostei," but the dispersal 

 of the " Ganoid " fishes has necessitated a new classifica- 

 tion, with the result that the familiar word " Teleost " may 

 in the future be " missed from its accustomed hill " in all 

 classifications of fishes. The Plaice (Pleuronectes platessa) 

 is the most familiar example in British seas of a group 

 having an almost universal geographical distribution. It 

 is the typical member of this group, which has long been 

 known as the Pleuronectidse, a family of fishes belonging 

 to the sub-order Anacanthini. In recent times this 

 family has itself received sub-ordinal rank, and has been 

 termed the Heterosomata, being divided into two families 

 (the Pleuronectidse and Soleidae) and six sub-families, 

 containing a large number of genera and species. The 

 principal diagnostic characters of the group are the torsion 

 which the anterior region of the skull undergoes during 

 development, and the modification and use of the left side 

 as the under side of the body. The lateral compression of 

 the body is paralleled among other Teleostean fishes, but 

 the [apparent] presence of both eyes on the right or left 

 side of the body is a unique feature. 



The nearest relatives of the Pleuronectidse among the 

 Teleosts are the Gradidse, and, curiously enough, these two 

 families afford the major portion of the fishes used as food 

 by man. The striking differences in general body form 

 and habits between the Plaice and Cod (typical examples 

 of the two groups) form a marked instance of how external 

 differences may coincide with deep seated morphological 

 similarity. The Plaice is a fish which is sluggish in its 

 movements, and has a very limited range of migration. 



