the Position of the Eyes in Pleuronectidge. 379 



is viewed from its right side. The right eye is rather more 

 round in outline — that is, a little shorter and broader than the 

 left. This diiference is caused by its being under pressure, 

 wedged in as it is between the anterior frontal bones and the 

 dorsal fin; and the pressure causes the interspinous part of the 

 dorsal fin to form a small slightly upward-curved projection 

 above the eye. 



The smallest of three young of Pleuronectes jylatessa, which 

 I have also before me, has a length of only 10 millims., the 

 measure being taken as above. The ramifications of the mu- 

 ciparous canal cannot be discovered on the left (blind) side of 

 the head, whilst on the right side they are just traceable as 

 indistinct lines ; but no ducts are to be distinguished. The 

 whole head has a clumsy, toad-like shape, the parts of the 

 mouth presenting soft swelling outlines. The right eye stands 

 over the beginning of the lower third of the maxillary bone. 

 The left eye stands at the top of the head, so much inclined to 

 the right, that from the left side only a trifle more than one- 

 third of the pupil can be seen ; it stands in front of the dorsal 

 fin, so that the front ray of the latter is just behind the end of 

 the left and beginning of the middle thirds of the eye. 



On comparing with this the head of another young P. pla- 

 tesstty which measures 14 millims. in length, one observes at 

 once that the head has a somewhat less clumsy appearance, and 

 the ramifications of the muciparous canal are more clearly ob- 

 servable. The position of the left eye is so far changed that 

 when the head is viewed straight from the left side the pupil 

 is invisible, only a part of the iris can be seen ; and its position 

 with regard to the dorsal fin is at the same time altered so that the 

 latter touches the left margin of the eye, which cannot now be 

 described as entirely situated in front of the dorsal fin ; for the 

 foremost ray of the fin stands at the side of the eye, a little in 

 advance of the extreme posterior margin of the eye. When, 

 therefore, the fish is examined from the blind side, the outline 

 of the eye cannot be followed in its whole extent, the posterior 

 extremity being hidden from view behind the first ray of the 

 dorsal fin. 



In a third young P. platessaj measm-ing 40 millims. in 

 length, the head presents almost the same pointed shape as in 

 the adult fish. The ramifications of the muciparous canal are 

 much more distinct, and the ducts are easily distinguished on 

 the prteopercular branch on both sides, as well as on the infra- 

 orbital branch on the blind side. The right eye stands over 

 the lower extremity of the upper jawbone; but, in spite of this 

 more depressed position of the right eye, the distance between 

 the eyes is nevertheless only a little greater than in the speci- 



27* 



