614 FISHES. 



are viviparous ; and to facilitate copulation the anal fin of 

 the adult male of many species is modified into a copulatory 

 organ, which is probably (partially at least) introduced into 

 the vulva of the female ; but it is uncertain whether it serves 

 to conduct the semen, or merely to give the male a firmer 

 hold of the female during the act. Also secondary sexual 

 differences are developed in the Cyprinodonts ; the males are 

 always the smaller, sometimes several times smaller than the 

 females, quite diminutive ; and they are perhaps the smallest 

 fishes in existence. The fins generally are more developed 

 in the males, and the coloration is frequently different also. 

 Some species are carnivorous ; others live on the organic 

 substances mixed with mud. Fossil remains have been found 

 in tertiary strata, all apparently referable to the existing 

 genus Cyprinodon ; they occur near Aix in Provence, in the 

 marl of Gesso, St. Angelo, in the Brown coal near Bonn, 

 near Frankfort, and in the freshwater-chalk of Oeningen. 

 In the latter locality a Poecilia occurs likewise. 

 The genera can be divided into two groups : 



I. Cypeinodontid^ Carnivoe^e. — The hones of each ramus 

 of the mandible are firmly united ; intestmal tract short, or 

 hut little convoluted. Carnivorous or insectivorous. 



Cyprinodon. — Cleft of the mouth small, developed laterally 

 and horizontally. Snout short. Teeth of moderate size, incisor- 

 like, notched, in a single series. Scales rather large. Origin of 

 the anal fin behind that of the dorsal in both sexes, both fins 

 being larger in the male than in the female. Anal not modified 

 into a sexual organ. 



Seven species occur in the Mediterranean region, all of 

 which seem able to live in briny springs or pools, the water 

 of which contains a much greater percentage of salts than 

 sea-water, as the brine-springs near the Dead Sea or in the 

 Sahara. They are as little affected by the high temperature 

 of some of these springs (91°), for instance of that at Sidi 



