UNUSUALLY COLOURED THORNBACK 25 



Wind E. by S., fine, very light. They were at first mistaken 

 for Teal flighting over. This appears to be the commonest 

 Grebe in Shetland; but none observed in 1892. 



LITTLE GREBE (Podicipes fluviatilis}. Only one observed on Loch 

 Spiggie, 26th October 1891. None seen in 1892 to date of 

 3oth October. 



AN UNUSUALLY COLOURED EXAMPLE OF THE 

 THORNBACK (RAIA CLAVATA, LINN.) 



By R. H. TRAQUAIR, M.D., F.R.S. 



Keeper of the Natural History Collections in the Museum of Science 



and Art, Edinburgh. 



PLATE I. 



A SHORT time ago Mr. Charles Muirhead, Edinburgh, 

 presented to the Museum a Thornback caught to the east of 

 the Isle of May, the unusual colouration of which renders it 

 worthy of being recorded and figured. 



The fish is a female and measures 33^- inches in length 

 from the tip of the snout to the termination of the tail, and 

 in breadth 2 !- inches across from angle to angle of the pectoral 

 fins. In general form and in its dentition and dermal arma- 

 ture it is a perfectly typical example of Raia clavata, Linn., 

 but in its colouration strangely aberrant. 



The ground colour of the upper or dorsal surface is white, 

 mottled all over with blotches of dark gray and small spots 

 of black so as to give it a most extraordinary appearance. 

 The colour of the ventral surface is quite normal. 



Protective resemblance is generally accepted as the lead- 

 ing motive in the tinting of the coloured surfaces of flat ground 

 dwelling fishes, whether Raiidas or Pleuronectidas, thus we 

 have ordinarily various shades of brown or brownish yellow, 

 mottled so as to imitate more or less the appearance of a 

 muddy, sandy, or gravelly sea-bottom. But it is hard to 

 imagine what manner of sea-bottom the colour of this Thorn- 

 back could imitate, unless it were a surface of white chalk, 

 scattered over with dark pebbles ! 



The only explanation which occurs to my mind is that 

 this colour-sport may be a case of partial albinism. 



