ON THE NU]\[RER AND ARRANGEMENT OF THE 

 BONY PLATES OF THE YOUNCI JOHN DORY. 



By L. W. BYRNE. 



The accompaiiying notes were inade some years ago at thc Suggestion of my 

 fricnd Mr E. \V. L. Holt*. It had beoii niy iiitentiDU to make thein more com- 

 plete by the e.xaniitiation of adult spocimoiis tVom the saiiie locality, but for thi.s I 

 have had neither time nor niaterial and they are only published now as throwing, 

 perhaps, a little light upon one of the natural causes which may have assisted in 

 the derivation of tlie asynimetrical Pleuronectidae froui some more normally- 

 formed member of the group Zeorlionibif. 



The material e.Kamined consi.sted of 250 specimens of Zeus ßibev from 2f to 5 

 inches in length and proliably about a year old, eaptured by Piymouth trawlers. 



For convenience the bony plates are in theso notes expressed by fonr letters or 

 numbers as follows : — 



Dorsal 



Left g I -^Q Right 



Ventral 



9 17,. 

 the diagram ,, rTT; , for instance, representing a speeimen with 9 left and 7 right 

 o I 10 



dorsal plates, and 8 left and 10 right anal plates. In the Plate dp . 1 indicates the 



Position of the anterior dorsal and ap . 1 of the anterior anal plate. 



The most striking peculiarity of the 250 specimens taken as a whole is the 

 very small proportion in which tiie plates are perfectl)' symmetrical in arrange- 

 ment, tlie number of plates on the left side equalling those on the right in both 

 the dorsal and anal series ; this arrangement may be e.xpressed by the diagram 



n\ n , • vk 1 .k 1 f fV 8 1 8 9 I 9 



— — wnere n is eitner less than, eonal to, or inore than m, e.g. „-r-?:, „ . „ , 

 m \ m ^ 9 I 9 9 I 9 



* See Bmilenger: Ann. Mag. Nat. Hist., Series 7, x. p. 303, 1902, where the ciicurastance.'i which 

 led Mr Holt to make this .Suggestion are explained. For the material used I am iiidebted to him and to 

 the Marine Biological Association. 



t Boulenger : loc. cit. p. 295. 



15—2 



