The Primitive Fishes 273 



hernia — so admirably monographed by Fritsch — all pro- 

 claim deposit in marshy areas, whose depth of water varied 

 from time to time. 



So far as present evidence then carries us, it can be said 

 that not merely the very primitive groups of fishes in Upper 

 Silurian and in Lower to Mid-Devonian time were wholly 

 freshwater, this condition persisted well into Upper Old 

 Red or Devonian times. But a marked distributional 

 change seems then to have begun. And one may here 

 record Chamberlin's words (5:11:535) as to the fish 

 life of the Devonian-Carboniferous. "Fish appear to have 

 first effectually invaded the open sea in the Devonian period, 

 but during that period true marine fishes seem to have been 

 inferior in number and variety to those of the inland 

 waters. But by the middle of the Mississippian (Calcifer- 

 ous or Lower Carboniferous: author) period the marine 

 fishes had made such relative progress that they were in 

 unquestioned supremacy, while the freshwater forms had 

 notably declined, if we may trust the record." The first 

 part of this statement is apparently correct; as to the 

 second the Mississippian and Carboniferous anadromous 

 fishes were fewer than the freshwater, a!nd the purely 

 marine ones were decidedly in the minority, as already in 

 part explained. But, as will be emphasized later, the 

 striking feature is that in late Carboniferous or in Permian 

 times most or possibly all of these marine migrants had 

 died out for some reason (p. 286). 



As bearing on the entire question of the possible distri- 

 bution in time and space of the great elasmobranch group, 

 it may be well now to ascertain whether any exact evidence 

 exists as to the environment of the group up till at least 

 Triassic times. A note of caution given by Wheelton Hind 

 deserves here to be quoted {106). "The marine or fresh- 

 water origin of much of the Coal Measures has always been 

 a disputed point. Palaeontologists have hesitated to 

 affirm the latter on account of the almost universal occur- 

 rence throughout the coal fields of Europe, in some few 

 beds at the base of the Coal Measures, of Unlo-like shells 

 with typically marine forms; but It Is questionable whether 

 the presence of the two forms together Is not apparent only. 



