384 Evolution and Distribution of Fishes 



mediate between Percopsidae and more recent Berycidae. 



The hypothesis may be of very small value, but if one 

 were to accept that such a primitive and intermediate type 

 or types existed in northern S. America, and that these then 

 spread along the northern coastal edge of the south-Atlantis 

 continent in mid or early late-Cretaceous times, such would 

 satisfactorily explain the distribution alike of fossil and of 

 recent genera. 



When one compares the Cretaceous and Eocene fossil 

 forms with living genera, it is seen that progressive evolu- 

 tionary modification in the fins is effected in a manner which 

 closely parallels that seen in the Centrarchidae, Percidae, 

 and Serranidae. Thus in Sphenocephalus and Acrogaster 

 the dorsal fin is continuous and shows only 3-5 spines with 

 10-12 soft rays. But steady increase in the number of 

 spines, and resulting lobed or ultimately divided condition 

 of the dorsal fin occurs, till in Myripristis and Holocentrum 

 it consists of 10-12 spiny also i soft anterior ray, and 

 II - 16 soft posterior dorsal rays that make up the two- 

 lobed structure. In like manner also the anal fins become 

 increased in their spine-rays but reduced in their soft rays. 



The swim-bladder has persisted in at least many of the 

 Berycidae, but usually as a simple sac, that rarely has an 

 open duct as in Beryx. This persistence may be ex- 

 plained in part by the fishes having become largely deep- 

 sea types, where oxygenation may be sluggish. 



The Pempheridae and Monocentridae appear both to 

 be derivative families from the last. They are composed 

 of a few genera and species of wide distribution over 

 tropical seas. 



The highly modified marine group of families that 

 Boulenger has called the Zeorhombi shows closest affinity 

 with the Berycidae, but must early have diverged — probably 

 in the late Cretaceous — from the more condensed and com- 

 pressed representatives of that group. The three families 

 that compose it are the Amphistiidae, the Zeidae, and the 

 Pleuronectidae or familiar flat-fishes. Practically all of 

 these are marine, but some species of the last-named family 

 inhabit eastern rivers. Thus while most species of Synap- 

 tura live along the sea-coasts of the East Indies and China, 



