520 Evolution and Distribution of Fishes 



spoonbill sturgeons, of which Crossopholis and Pholidurus 

 are ancient types, Polyodon and Psephurus are recent. Their 

 distribution resembles that of the previous group. The 

 Holostei start with a few representatives like Acentropho- 

 rus in Permian time, and as a probable offshoot from more 

 ancient chondrosteans. They attain their climax however 

 in Mesozoic lakes, rivers, and swamps. All of the holo- 

 stean families had already become differentiated before 

 the period of the Lower Oolite, except the modern Lepido- 

 steidae and Amiidae. During the Triassic numerous genera 

 and species evolved and spread abroad from Connecticut 

 to France, Germany, Austria, N. Italy, S. Africa, India, 

 and East Australia. Such suggests some fairly easy and 

 connected line of travel, that linked up these geographic 

 centres. But there is considerable evidence to indicate that 

 N eorhombolepis , Belonostomus and some others became 

 marine livers, if the "Chalk" strata in which they occur are 

 truly sea deposits. Two holostean subgroups however 

 gradually became pronounced sea fishes. One of these, the 

 Pachycormidae, started with such large lithe freshwater 

 forms as some species of Pachycormus, Includes such a 

 transition genus as Hypsocormus and developed into types 

 like the marine Protosphyraena, that might well be called 

 the "ganoid shark" of the Chalk formation. 



The other of these, the Pycnodontidae started in the 

 Jurassic with freshwater genera like Gyrodus, but ulti- 

 mately they condensed, as they migrated seaward, into 

 short rounded marine fishes like Coelodus and PalaeohaUs- 

 tum of Upper Cretaceous and Eocene strata. 



The holostean group Amiidae seems to have descended 

 from the more ancient Eugnathidae during later Jurassic 

 time, and amid lakes or swamps of Central Europe. But 

 in late Cretaceous or early Eocene time and onward they 

 reached and rapidly multiplied in the extensive lake-river 

 system of N. America. Here the one species Amia calva 

 Is found. Reference Is next made to the Aspidorhynchldae 

 of the calcareo-bitumlnous rocks of Cerin and Solenhofen 

 — both probably of volcanic origin. The abundance of 

 fishes In the strata of Cerin evidently explains the mode of 

 origin of the "bituminous schists" of that region. A list 



