8 
seen five {uberosities or tubercles. These may vary con- 
siderably both in prominence and in number. Sometimes 
only four are present, and also any one or all may be in 
duplicate. As a rule, however, five occur, situated as 
follows:—1l and 2 on the right frontal, 3 on the right 
sphenotic, 4 on the right pterotic, and 5 on the zight post- 
temporal (see figs. 1 and 21). 
Tur ASYMMETRY OF THE Puatce.t 
The most striking feature in the external appearance 
of the plaice, and also the most interesting in its anatomy, 
is the apparent presence of both eyes on the upper, right, 
or ocular side. But this is not the only respect in which 
the head of the Plaice has undergone torsion. The jaw 
apparatus is also very asymmetrical, and in a different 
direction, for whilst the eyes are twisted towards the 
ocular side, the jaws incline towards the eyeless side. 
Now it must be obvious at the outset that the asymmetry 
of the jaws has been superimposed on that of the eyes, 
and is in fact a special adaptation to an already asym- 
metrical fish, living on the sea bottom, and lying on its 
left side. We may therefore leave this asymmetry to be 
described in its proper place, and confine ourselves to that 
of the eyes. 
The asymmetry of the Pleuronectide was first cor- 
rectly explained by Traquair in 1865. The question is 
beset with numerous difficulties, in the form of many 
secondary modifications tending to mask the true course 
of the original torsion. Traquair, however, in his now 
+ We have no space to refer to the extensive literature on the asymmetry 
of the Pleuronectide, especially as the work of Traquair covers most of the 
facts. We should like, however, to mention an interesting paper by Holt 
on an abnormal sole (P.Z.S., 1894, p. 482). 
