458 FOSSIL INVERTEBRATED ANIMALS. 



a higher degree of saUness. Now, the fact is, that one suppo- 

 sition is no more easy to be conceived than the other. 



On the whole, however, it must be observed, that it is not 

 the presence of very precise characters which enables us to 

 distinguish marine and fresh water shells. What usually fixes 

 the judgment respecting the last, is the recognized identity of 

 certain genera, or species, which have never been met with in 

 the living state but in the fresh water, and which have never 

 been found as fossils in marine formations. 



As to fossil remains in general, they have a greater or a less 

 analogy with what is now existing ; and this analogy is more or 

 less easy of verification. 



In plants, for instance, we easily distinguish the fossil wood, 

 the family of monocotyledon trees from that of dicotyledons ; 

 but it is not so with the genera. This difficulty, however, may 

 proceed from the contexture of woods, in the living state, not 

 having been sufficiently studied. 



The study of fossil stems, leaves, and fruits, has led to the 

 recognition of many genera, which have been distinguished by 

 M. Adolphe Brogniart. But there are many vegetable debris 

 which, as yet, no botanist has been able to refer to any thing 

 analogous m the existing state of the vegetable kingdom. 



We have already amply seen that the long labours of the 

 most illustrious comparative anatomist of the day have made 

 known a wonderful number of species of cetacea, of reptiles, 

 of birds, and of mammalia, many genera of which have disap- 

 peared from the surface of the globe, and, perhaps, have never 

 been known in the living state to man ; but of his remains, as 

 we formerly remarked, nothing has been found, nor yet of those 

 of the quadruma ; and, hitherto, it has only been in strata more 

 re^nt than those of .the coarse limestone that any fossil remains 

 of the mammifera have been discovered. 



We have also seen, with respect to fishes, that, being in a 

 great measure composed of soft organs, which have been de- 

 stroyed before petrifaction could have seized them, they have 



