510 Evolution and Distribution of Fishes 



in offensive action of some species of fish, also to the rich 

 stores of invertebrate food obtained in the sea. 



The Jurassic deposits that are mainly or wholly fresh- 

 water, and those that are marine, are distinguished, their 

 faunal types are compared, and fishes that show anadrom- 

 ous tendencies, or that become marine are noted. The pre- 

 vailing types of fish belonged to the dipnoan and ganoid 

 divisions. These wholly inhabited freshwater, or some 

 highly specialized ganoids were migrating seaward during 

 the later Jurassic. The marine elasmobranchs however 

 were again becoming dominant fish forms. 



The remarkable Solenhofen-Eichstadt "lithographic" 

 beds, and their strangely heterogeneous freshwater-marine 

 organisms are studied, while all of the species of fishes are 

 listed after Walther's extensive observations and records. 



In summing up the entire evidence for the period the 

 writer concludes "that all of the great groups of fishes 

 continued as inhabitants of the lakes, rivers and swamps 

 throughout the Jurassic and into the Cretaceous, except 

 for genera of the rays, the dog fishes, probably some of the 

 Pycnodonts like Coeiodus, also more doubtfully a few 

 'ganoid' genera like Hypsocormiis and Aspidorhyjichus. 

 But even the earlier species of Hybodus and Acrodus that 

 occur in the Lias and Stonesfield Slate, were still largely 

 lake dwellers." 



But from Souabe he cites one amongst such cases, where 

 a mixing of freshwater and marine rocks along with the 

 organisms peculiar to these, has caused loose and erroneous 

 statements to be made. 



The extension of Jurassic (Upper Liassic) beds into 

 Central Siberia, India, and Australia, also the presence in 

 these of freshwater fish genera and other organisms, that 

 are identical in some cases with those found in Western 

 Europe, are proofs that considerable connection now existed 

 between the great northern and southern continents of that 

 time. 



Chapter 7. The physical and biological environment of 

 fishes during Cretaceous time. 

 This is viewed as an important stage in the earth's 

 history. "For all accumulated evidence combines to show 



