' PALSEONTOLOGICAL SERIES:' 79 



Of the great series of "worms, 5 ' only the tube-makers have left 

 actual remains ; the others are known only by their tracks, while of 

 nny that may have lived on the land there is no evidence. 



The Echinoderms, because of their hard parts, are well represented 

 in all their orders, except the Ilolothurians, where the calcareous 

 structures characteristic of the class are at a minimum. 



The Crustacea, being mostly aquatic, and in virtue of their hard 

 shells, are fossilised in great numbers. 



The Arachnida and the Insects, owing to their air-breathing habit, are 

 chiefly represented by chance individuals that have been drowned, or 

 enclosed within tree-stumps and amber. 



The Molluscs and Brachiopods are perhaps better preserved than 

 any other animals, since nearly all of them are possessed of a shell 

 specially suitable for preservation. 



Among the Vertebrates some of the lowest are without scales, teeth, 

 or bony skeleton ; such forms have therefore left almost no traces. 



Fishes, which are usually furnished with a firm outer covering, or 

 with a bony internal skeleton, or with both, are well represented. 



The primitive Amphibians were furnished with an exoskeleton of 

 bony plates, and are fairly numerous as fossils. The bones and teeth 

 of the others have been fossilised, though more rarely. Of some the 

 only record is their footprints. 



The traces of Reptilia depend upon the habits of the various orders, 

 those living in water being oftenest preserved, but the strange flying 

 Reptiles have also left many skeletons behind them. 



Of the Birds, the wingless ones are best represented, and then those 

 that lived near seas, estuaries, or lakes. 



The history of Mammals is very imperfect, for most of them were 

 terrestrial. But the discoveries of Marsh, Cope, and others show how 

 much may be found by careful search. The aquatic Mammals are 

 fairly well preserved. 



" Palaeontological series." -In spite of the imperfection 

 of the "geological record," in spite of the conditions un- 

 favourable to the preservation of many kinds of animals, it 

 is sometimes possible to trace a whole series of extinct 

 forms through progressive changes. Thus a series of fossil- 

 ised fresh-w r ater snails (Planorbis] has been worked out ; the 

 extremes are very different, but the intermediate forms link 

 them indissolubly by a marvellously gradual series of transi- 

 tions. The same fact is w r ell illustrated by another series of 

 fresh-water snails (Paludina^ Fig. 35), and not less strikingly 

 among those extinct Cuttle-fishes which are known as 

 Ammonites, and have perfectly preserved shells. Similarly, 

 though less perfectly, the modern crocodiles are linked by 

 many intermediate forms to their extinct ancestors, for it is 

 impossible not to call them by that name. In short, as 



