2 14 Evolution and Distribution of Fishes 



a massive fluviatile formation corresponding to the English 

 Wealden, and divisible into two groups: First, Deister 

 Sandstone (150 feet), like the Hastings sand of England, 

 consisting of fine light-yellow or grey sandstone (forming 

 a good building material), dark shales, and seams of coal 

 varying from mere partings up to workable seams of three, 

 and even more than six feet in thickness. These strata are 

 full of remains of terrestrial vegetation [Equisetiim, 

 Baiera, Oleandridium, Laccopteris, Sagenopteris, Anomo- 

 zamites, Pterophyllum, Podozamites and a few Conifers) 

 also shells of freshwater genera {Cyrena, Fivipartis, 

 Cyprids), and remains of Lepidotus and other fishes; 

 Second, Weald Clay (65-100 feet), with thin layers of 

 limy sandstone {Cyrena, Unio, Vtviparus, Melania, Cypris, 

 etc.)." It will be observed that there is no indication of 

 estuarine or semimarine organisms in this list, and so we 

 would regard all, as in the case of the English Wealden, as 

 of lacustrine origin. 



But with the close of the Wealden period, or in some 

 places during Neocomian times, wide areas that are now 

 dry land, became submerged to a greater or less degree 

 below sea level, and over these areas the Upper Greensands 

 and zones of chalk were deposited. The invertebrate life 

 at that time must have been extremely rich, as every list of 

 marine fossils testifies. But mixed amongst these in increas- 

 ing quantity and variety are the remains of fishes. These 

 consist chiefly of teeth, jaws, scales, or spines — more rarely 

 of the partial or entire body — of selachian, cestraciont, 

 chimaeroid, or more rarely of teleostean types. These, 

 descending from the lakes or rivers, during Triassic and 

 still more during Jurassic times, became now the prevailing 

 groups of marine fishes, and likewise from their offensive 

 and defensive armature, must have proved formidable an- 

 tagonists to the medusae, the cephalopods, and the evolving 

 higher marine crustaceans of that time. 



But from such primitive Jurassic freshwater genera 

 of pycnodonts as Mesodon, Athrodon, and Microdon de- 

 rivative marine genera like Pycnodiis, Anomoeodiis, and 

 Coelodus, arose in later Jurassic times, and reached a 

 climax in the mid or upper Cretaceous period. Then they 



