222 METAMORPHOSIS AND 



side. The Flounders of the Mediterranean are much 

 less spinous than those of the North Sea or Channel. 

 The Dab (P. limanda) occurs on the American 

 coast in a local form called Limanda ferruginea, 

 and in the North Pacific there is a rougher form 

 called jL. aspera. In these three species therefore, 

 apart from mutations, the northern forms all show a 

 greater development of spines on the scales. Whether 

 this is an effect of colder temperature it is difficult 

 to say. It is possible that the difference is due to 

 external conditions, of which lower temperature of 

 the water is the most obvious, and it may be that 

 these conditions have a greater effect on the male 

 than on the female in the Plaice. 



Sexual differences in scales, which have a function 

 in the relations of the sexes, occur in a few other 

 fishes, and these can be attributed with good reason 

 to mechanical stimulation. For example, in the 

 Rajidae among Elasmobranchs the males possess 

 on each ' wing ' or pectoral two series of large, 

 recurved, hooked spines. It has been stated,^ 

 apparently by Yarrell, that these spines are de- 

 veloped only in the breeding season. It is doubtful 

 if there is any marked breeding season in these fishes, 

 but it is probable that the spines are absent in the 

 immature male, as it is known that in Raia clavata 

 the adult male has sharp pointed teeth, while the 

 young male and the female at all ages have broad 

 flat teeth. It is supposed that the spines and perhaps 

 the sharp teeth are used for holding the female, but 

 it seems equally probable that these structures are 

 really used by the males in fighting with each other. 

 The habits of these marine fish have not been much 



1 Darwin, Descent of Man (2nd edit., 1885), p. 331. 



