38 INTRODUCTION. 



riers ; and consequently, taken as a whole, the fauna of this 

 series may be regarded as one and indivisible. The condi- 

 tions, nevertheless, which obtained in different parts of this 

 area were very different; and, as a necessary result, certain 

 groups of animals flourished in certain localities, and were 

 absent or but scantily represented in others. In the deeper 

 parts of the area we have an abundance of Corals, with Crinoids, 

 and at times Foraminifera. In the shallower parts of the area 

 there is, on the other hand, a predominance of forms which 

 affect water of no great depth. Still, there is no difference in 

 point of time between the deposits of different parts of the area; 

 and in order to obtain a true notion of the Lower Carbonifer- 

 ous fauna, we must add the fossils derived from one portion of 

 the area to those derived from another. 



In many cases, however, we are acquainted with but one 

 class of deposits belonging to a given period. We may have 

 the deep-sea deposits of the period only, or we may know no- 

 thing but its littoral accumulations. In either case it is clear 

 that there is an imperfection of the palseontological record ; 

 for we cannot have even a moderately complete record of the 

 marine animals alone of a particular period, unless we have 

 access to a complete series of the deposits laid down in the 

 seas of that period. 



IV. SUDDEN EXTINCTION OF ANIMALS. Whilst there can 

 be little doubt but that the changes in animal life indicated by 

 Geology were in the main gradually effected, there still remain 

 cases in which individuals seem to have been suddenly de- 

 stroyed in great numbers, and others of a more obscure nature 

 in which allied species succeed one another with an inexpli- 

 cable rapidity. As an example of the first class of cases, we 

 may take the great Marine Reptiles of the Lias, which often 

 exhibit indications of having met a sudden death, whilst they 

 show no traces of mechanical injury. It has been suggested by 

 Sir Charles Lyell, with great probability, that the sudden death 

 of marine animals, in these and other similar cases, might have 

 been caused by the sudden "periodical discharge of large 

 bodies of turbid fresh water into the sea." 



As an example of the second class of cases which more 

 especially bear upon the present question we may take the 

 existence in the Lias of zones characterised by particular species 

 of Ammonites. These zones are usually of small thickness, 

 and the Ammonite characterising each is usually confined to 

 that particular horizon ; whilst several of these zones have been 

 found to be persistent over very wide areas. As we know of 

 no reason why one species of Ammonite should flourish where 



