8 



seen five tuberosities or tubercles. Tliese 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: — 1 and 2 on the rig-bt frontal, 3 on the right 

 sphenotic, 4 on the right pterotic, and 5 on the right post- 

 temporal (see figs. 1 and 21). 



The Asymmetry of the Plaice.! 



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 difierent 

 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 Pleuronectidie 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 



t We have no space to refer to the extensive literature on the asymmetry 

 of the Pleuronectidse, 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. 432). 



