190 SEXUAL DIMORPHISM 



developed in the plaice of the English Channel, but become 

 more common towards the north, namely, north of the Dogger 

 Bank and on the south coast of Iceland. In the Baltic the 

 spinulation is very strongly developed. 



In Arctic waters there occurs a species which must be 

 considered as the geographical representative of the Plaice, and 

 the latter is absent from the same waters except where the 

 ranges of the two forms meet one another. This is the form 

 originally described as Pleuronectes glacialis by Pallas. It is 

 distinguished from the Plaice chiefly by the fact that there are 

 no prominent tubercles on the lateral ridge of the head behind 

 the eyes ; the ridge is rough and nearly continuous. Pallas' 

 specimens came from the Kara Sea and the mouth of the 

 river Obi, and he did not mention any sexual differences ; 

 the upper side had rough scales. On the east coast of the 

 United States, where the Plaice is absent, occurs a species 

 considered to be identical with the glacialis of Pallas, in 

 which the American ichthyologists have found that the males 

 have spinulated or ctenoid scales, while in the females the 

 scales on the upper side are smooth, with the edges entire. 

 On the coast of Alaska specimens have been taken referable 

 to the same species, but the females were spinulated. We 

 have no information concerning sexual differences in specimens 

 obtained from the north coasts of Europe, Asia, and America. 

 It is certain, therefore, that a sexual difference similar to that 

 existing in the Plaice occurs in the PL glacialis of the east 

 coast of North America, while in specimens of the same form 

 from more northern localities and from the coast of Alaska, 

 the spinulation appears to occur in the females as well as 

 in the males. 



Special developments of spinulation have not been ob- 

 served to occur in the males in other species of Pleuronec- 

 tidse, and there seems therefore little reason for supposing 

 that in the Plaice the character is a mere expression or result 



