362 A HISTORY OF FISHES 



During the Jurassic period an offshoot of Herring-hke fishes, 

 the Plwlidophoridae, arose from the Semionotid stem, and in 

 general form and in the shape of the fins these bear a markedly 

 close resemblance to the modern Herring-like fishes. The 

 presence of minute teeth in the jaws indicates that these extinct 

 forms were probably plankton-feeders. They gave rise towards 

 the end of the Jurassic to another family, the Leptolepidae, the 

 first Bony Fishes with a homocercal tail. These were very like 

 the existing Ten-pounders {Elops), a genus abundantly repre- 

 sented in Cretaceous times, but now reduced to a few species, 

 which are to be regarded as the most primitive of living Bony 

 Fishes of the modern type. The Leptolepidae were the earliest 

 members of the first of the great orders of modern Bony Fishes, 

 namely, the Isospondyli, a group which includes all the Herrings, 

 Salmon, Trout and their allies, as well as the Osteoglossids, 

 Mormyrids, and other related forms. During the Cretaceous 

 period this order underwent considerable evolution, and several 

 important offshoots made their appearance, including the 

 earliest members of the order Iniomi and the first Eels {Apodes), 

 as well as the first spiny-rayed fishes in the form of primitive 

 Berycoids. There is a slab of rock in the British Museum 

 containing the remains of numerous individuals of one of these 

 early Berycoids named Hoplopteryx^ which are preserved in a 

 most extraordinary manner, and seem to represent part of a 

 shoal suddenly overwhelmed by some catastrophe. The fishes 

 are lying one upon the other, their bodies frequently twisted 

 or contorted in various ways, the mouths and gill-covers gaping 

 wide, and the fins erect, conditions which suggest sudden 

 asphyxiation (PI. VI). 



No true Perch-like fishes make their appearance until towards 

 the end of the Cretaceous, but from this time onwards their 

 evolution is very rapid. It will be impossible to follow the 

 history of modern Bony Fishes any further here, especially as 

 their geological history is often far from complete. It is known 

 that most of the modern families began somewhere in the early 

 part of the Eocene, for remains of Scorpion-fishes, Sucker-fishes, 

 File-fishes, Mackerels, Sword-fishes, and Anglers have been 

 found in the rocks of that period. 



