of Mount Lebanon. 243 



different classes. We see, at a given moment, a certain class be- 

 come modified to intensity, while a certain other class preserves 

 its general physiognomy intact, to become subject to a similar 

 process at another period. 



The class of fishes is remarkable in this particular*. The last 

 extensive modification it has undergone corresponds to the trans- 

 ition between the Jurassic and Cretaceous periods. Now, with 

 regard to most classes, this transition is relatively of little im- 

 portance. We see the Jurassic reptiles continuing a great many of 

 their types in the Cretaceous period, whilst the transition between 

 this and the Tertiary period is marked by the most striking 

 changes of form. We see the Mollusks, the Echinoderms, and 

 the Polypes of the cretaceous seas reproducing to a great extent 

 the types of their Jurassic predecessors. If we were to seek the 

 epochs when the greatest modifications in each of these classes 

 have taken place, we should never find them in the interval be- 

 tween the Upper Jurassic and Neocomian periods. 



The importance of the change which took place at the termi- 

 nation of the Jurassic period has already been dwelt upon suffi- 

 ciently by M. Agassiz. Our learned friend has laid stress in 

 particular on the apparition somewhat suddenly at the com- 

 mencement of the Cretaceous period of the most perfect group 

 of fishes, the Teleostei, which form the large majority of the 

 population of the modern seas. Saving a certain measure of 

 restriction imposed by later researches upon the generality of 

 this assertion, the fact has in the main received every confirma- 

 tion. It gives a particular importance to the study of the Cre- 

 taceous fish-faunas, since these faunas are the origin and, in 

 some measure, the earliest expression of our present existing 

 ones. It is interesting to follow the gradual series of modifica- 

 tions through" which they have passed, to note the earliest repre- 

 sentative types, the forms which have continued most constant, 

 and those which have been the last to appear. The most gene- 

 rally adopted classification of fishes is that of J. Miiller. Of the 

 six subclasses established by him, three have no fossil represen- 

 tatives {Leptocardii, Cyclostomi, and Dipnoi) ; the three others 

 alone enter the domain of the palfeontologist. 



Among these three subclasses, the Elasmohranchi retain the 

 same general characters which they have presented throughout 

 all time. This is the group which has undergone the least mo- 

 dification. It is not represented very abundantly at Lebanon ; 



* Prof. Heer has just called attention to a perfectly similar fact in the 

 history of the vegetable kingdom, ' Les Phyllites cretaces dii Nebraska ' 

 (Extrait des Mem. de la Soc. helvet. des So. Nat. 186(5). He has shown 

 that the Upper Cretaceous flora is quite different from the Jurassic flora, 

 and allied rather to the Tertinrv flora. 



17* 



