During Triassic and Jurassic Periods 195 



beds. But the prominent feature is the varied lot of am- 

 phibious to marine reptiles that invaded or often swam on 

 the waters. A like condition is revealed by a considerable 

 thickness of the Stonesfield Slates and the Kimmeridge 

 clay. It is in this latter zone also that the remarkable Solen- 

 hofen and Cerin slates occur that are dealt with in consider- 

 able detail below, and the organisms of which give us a 

 fuller and more graphic idea of the life of this period, 

 than do any other known rocks of the entire geologic scale. 



In some upper Portland beds, and in Purbeck sections 

 of central England, Brodie and others obtained rich organic 

 remains from a set of slaty limestones. In addition to land 

 plants, cyprids, a great variety of insects, several genera 

 of freshwater molluscs, and an important series of fish re- 

 mains were assembled. While a considerable number of 

 elasmobranch fishes like Hybodus and Acrodus still lived 

 in the lakes, the dipnoan and ganoid genera were greatly 

 more numerous, and in some localities their remains are 

 piled on each other. So in many places over the European 

 continent from England eastward, bituminous rocks that 

 contain decomposed products of these fishes are known. 



Over the European continent extensive masses of 

 Jurassic freshwater beds have been treated of by Oppel 

 (7^7), by Quenstedt {148: i), also by Credner, Meyer, 

 PIctet, and others. But attention may now be given to 

 the remarkable Solenhofen-Eichstadt slates. The fish- 

 fauna of these was studied in succession by Agassiz, A. 

 Wagner, V. Meyer, Winckler, and V. Reis. But the 

 most complete and recent study is that of I. Walther en- 

 titled "Die Fauna der Solenhofener Plattenkalke" {149'- 

 135). The material composing these slates is an extremely 

 fine-grained and hard limestone that has attained inter- 

 national repute as yielding the finest grade of lithographic 

 stone. The deposit occurs in somewhat discontinuous 

 masses along the Wiesenthal of Bavaria, and varies from 

 20 ft. to nearly 80 ft. in thickness. It is readily divisible 

 into a series of layers {FUnze of Walther) that vary from 

 thin plates to blocks which average 10-15 cm. in thickness. 

 These split apart readily from each other, owing to delicate 

 films of deposit that are usually of darker and more earthy 



