FOSSIL MAMMALIA. * 



pursued with that degree of attention which it deserves. In- 

 volving infinitely greater difficulties than that of the conchology 

 of existing species, much fewer of the former than of the latter 

 have been discovered. It is yet the opinion of some eminent 

 naturahsts that the number of ancient species may equal, if not 

 exceed, that of the modern. They are led to this conclusion 

 by considering that the latter appertain but to a single era, 

 while the former are attached to many successive periods in 

 which animals of different descriptions have been abundantly 

 produced. 



It is but seldom that we meet with shells in the fossil state 

 of species perfectly analogous to those which now exist. There 

 is scarcely any exception to this but in the case of the fossils 

 imbedded in the low hills of the Apennine range, of which a 

 considerable number are found in a living state in the neigh- 

 bouring Mediterranean. It is, however, remarkable that a 

 number of mollusca and marine polypi found in this sea in 

 abundance are not discovered in the fossil state, as, in like 

 manner, fossil species are found in the Apennines that no longer 

 exist. This want of perfect similarity is by no means sur- 

 prising when we find that even species of the same strata, and 

 of the existing seas, do not perfectly resemble when the habitat 

 differs. 



The remains of mollusca and zoophytes are much more nu- 

 merous than other * fossils, and the strata in which they are 

 found are sometimes changed into calcareous stone. They are 

 found in falun, in marl, in clay, and in gres or granulated 

 brownish quartz. Shells, nearly resembling those of our 

 marshes and streams, are found in the more recent strata. 



Between the strata composed of marine fossils, we sometimes 

 meet others containing terrestrial remains of animals or vege- 

 tables, which prove the settlement and return, at different 

 periods, of the sea and fresh water, and even between these 

 periods, the absence for a time of both, as it appears that the 



