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. Miller. Of the 
six subclasses established by him, three have no fossil represen- 
tatives (Leptocardii, Cyclustomi, and Dipnor) ; the three others 
alone enter the domain of the paleontologist. 
Among these three subclasses, the Elasmobranchi 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 erétacés du Nebraska’ 
(Extrait des Mém. de la Soe. helvét. des Se. Nat. 1866). He has shown 
that the Upper Cretaceous flora is quite different from the Jurassic flora, 
and allied rather to the Tertiary flora. 
ie 
