362 Evolution and Distribution of Fishes 



from the Scopelidae onward. For in some of these it is 

 fairly large, in others absorbed; in some of the Stephano- 

 berycidae it still exists; but in the most highly modified 

 Aleposauridae and Chirocentridae it has been wholly ab- 

 sorbed. 



The remaining freshwater families of the Haplomi form 

 a striking series, alike as to structure and distribution. The 

 Esocidae or Pike family, though only known back to the 

 Miocene period suggests in its distribution, a much more 

 remote ancestry. For while made up of but two genera, 

 Esox and Umbra, the former has gradually spread across 

 the entire northern hemisphere. Since several species of 

 Esox have been described from central European strata 

 of Miocene age; since the only two species of Umbra are 

 found one in Austria, and the other in the eastern United 

 States and Canada; and since species of Esox connect these 

 widely apart regions, it is highly probable that the group 

 spread from Europe across Asia and thence across N. 

 America to its eastern seaboard. The peculiarly modified 

 genus Dallia of eastern Siberia and Alaska, suggests ancient 

 derivation — as a side-line of organic evolution — from some 

 type allied to Umbra. It also indicates that Umbra itself 

 once had a much wider range. 



But the more evolved family Cyprinodontidae is even 

 more arresting. The group is first known from freshwater 

 Miocene beds of Central and South Europe in the form of 

 the genera Prolebtas (Fig. 40, p. 249) and Pachylebias. As 

 the figure shows, the former of these must have lived and 

 been killed in swarming shoals. Little trace is known of 

 them in more recent deposits till we reach present-day types. 

 Of these there are about 200 species, which are almost whol- 

 ly dwellers in rivers, lakes or swamp-lands of the United 

 States, of central and S. America, of all Africa, and a skirt- 

 ing south-western part of Asia on to Borneo. But in some 

 species, mainly belonging to Fundulus, a tendency toward 

 marine life is noted. Thus while most of the species are 

 truly freshwater, three occur in rivers and lakes of the 

 Eastern States, but frequently pass out to, and are abund- 

 ant along, the seacoast. Some species of Cyprinodon again 



