18 PALEONTOLOGY 



exceptions, every class of invertebrate animal is represented 

 by fossil remains. 



They consist of corals and shells, of the petrified skeletons 

 of star-fishes and sea-urchins, of the hard coverings of crabs 

 and insects, of the tracks and shelly habitations of worms, and 

 of impressions of surfaces, and casts of cavities, of organisms, 

 retained by the matrix after the animals had perished. 



The condition in which invertebrate fossils occur depends 

 on the nature of the matrix and other accidental circum- 

 stances ; for while some are scarcely altered in composition, 

 or even in colour, others are silicified or infiltrated with car- 

 bonate of lime. Some may be cleared by the action of acid 

 or exposure to the weather, and some require the chisel of the 

 mason or the mill of the lapidary for the proper exhibition of 

 their structure. 



Multitudes of recent species are fossilized in the newer 

 tertiaries whose history can be made out perfectly from living- 

 specimens ; but the number of these diminishes gradually in 

 each older stratum, while the proportion of extinct forms is 

 ever on the increase. No living species more highly organized 

 than a Ehizopod is found in the secondary rocks. Recent 

 genera extend further back in time ; indeed a few may be 

 recognised in strata of palaeozoic age, shedding a light on the 

 probable affinities and conditions of their associates. Many 

 of the smaller groups of genera, called families, disappear in 

 the secondary, and still more in the palaeozoic period, and are 

 to a limited extent replaced by groups which, no longer exist. 

 But as to the larger groups of Protozoa and of true inverte- 

 brate animals, it may be affirmed that every known fossil 

 belongs to some one or other of the existing classes, and that 

 the organic remains of the most ancient fossiliferous strata do 

 not indicate or suggest that any earlier and different class of 

 beings remains to be discovered, or has he* n irretrievably lost, 

 in the universal metamorphism of the oldesl recks. 



