22^ Evolution and Distribution of Fishes 



tural characters typical of certain families belonging to 

 the present-day dominant group of fishes — the Teleostei. 

 For while the last studied group of Pycnodontidae develop- 

 ed into an increasingly evolved but physiologically decadent 

 type of organisms, the pachycormid derivatives from a eu- 

 gnathid stock became increasingly lithe, active, adaptable 

 and numerous. Moreover, from their structural details 

 they seem to have been the Mesozoic progenitors of 

 certain groups of Teleostei which in surviving the tre- 

 mendous volcanic upheavals and changes of early Eocene — 

 or more likely upper Cretaceous — time, are now repre- 

 sented in descendants that are in part the living bony fishes. 



Already in beds of Upper Liassic age the oldest known 

 genera Sauropsis, Prosaiiropsis, Euthynotus, and Pachycor- 

 mus have left remains that indicate animals of a foot to 

 three feet in length, and which from the numbers preserved 

 on a single slab, must have been more or less social in habit 

 and abundant in individuals. Side by side with these also, 

 and only found as a rule in an argillaceous or a blue bitumin- 

 ous and typically nodular shale occur Ptycholepis, Lepidotus, 

 Tetragonolepis, Dapedius, Gyrosteus, and other genera 

 already treated of, as well as Leptolepis and others men- 

 tioned below (p. 344). Associated with all of these again, 

 are remains of insects, fossilized freshwater shells and 

 wood, but most conspicuously great saurian and crocodilian 

 remains, that together proclaim a combination of land and 

 freshwater as contributing centres of organic life. {240:- 

 396-400). These freshwater Upper Liassic beds are con- 

 tinued from West England across into Belgium, France, 

 and thence into Germany, thus indicating large lakes or 

 lacustrine swamps, that existed at no great distance from 

 the Upper Liassic seas of the period. Such lacustrine areas 

 probably originated first as arms of the sea, that by land 

 oscillation, or by formation of sand-bars, or both, became 

 inland lakes into which rivers emptied, and from which 

 rivers passed to the sea. 



It is somewhat peculiar that our knowledge is still 

 extremely limited regarding the Jurassic and early Cretace- 

 ous fish-life of all regions outside Europe. This seems the 

 more peculiar, when we remember how abundant and varied 



