FISHES 183 



Dr. Giinther. 1 In the male the mouth and interopercular 

 bones are fringed with spines, of which the female shows hardly 

 a trace. I cannot discuss this case, because I know nothing 

 of the habits of the fish, or the function of the spines. In 

 another species of the same genus soft tentacles are present on 

 the front part of the head of the male, absent in the female. 



Sahnonidw. Male salmon fight constantly and violently 

 with one another, and the turning up and enlargement of 

 the tip of the lower jaw appears to be the result of the blows 

 inflicted against the bodies of rivals. This growth of the 

 lower jaw, like the antlers of stags, is present only during 

 the breeding season. Darwin quotes Mr. Lloyd's testimony 

 that the temporary hook-like structure serves to strengthen 

 and protect the jaws when one male charges another with 

 wonderful violence. We have here a mechanical violence 

 and irritation which, as in the case of the antlers of stags, 

 is evidently capable of producing to some extent in the 

 individual the modification of structure, whose evolution we 

 have to explain. 



Cyprinodontidce. In many of the species of this family 

 very distinct sexual differences occur. Darwin mentions 

 only two species, namely Mollienisia petenensis, in the male 

 of which the dorsal fin is enlarged and marked with ocelli, 

 and Xiphoplwrus Hellerii, in the male of which the inferior 

 margin of the caudal fin is developed into a long fila- 

 ment. S. Garman has recently published a memoir 2 on the 

 Cyprinodonts, in which the characters of all the species are 

 described, but the habits of the fish in life are not discussed 

 in detail. In many of the genera the front portion of the 

 anal fin is elongated and modified to form an intromittent 

 organ for the milt, so that fertilisation is internal, and the 

 young are born alive. In Mollienisia the anal fin of the 



1 Proc. Zool. Soc. 1868, p. 232. 

 2 Mem. Mus. Comp. Zool. Ifarv., vol. xix. No. 1. 



