INTRODUCTION. 
Tue 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 Pleuronectide, 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 Pleuronectide and Soleide) 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 Pleuronectide among the 
Teleosts are the Gadide, 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. 
