76 PAST HISTORY OF ANIMALS. 



are very abundant in the rocks of many ancient seas. But, strange as 

 it may seem, some beautiful remains of jellyfish have been discovered. 



Of the great series of "worms," only the tube makers have left actual 

 remains, the others are known only by their tracks, while of any 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 Holothurians, where the calcareous structures 

 characteristic of the class are at a minimum. 



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

 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. Those living in fresh 

 water have left footprints as traces. 



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. 



' l Palceon tologica I Series. ' ' 



In spite of the imperfection of the "geological record," 

 in spite of the conditions unfavourable 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 fossilised fresh water snails (Planorlns} has 

 been worked out ; the extremes are very different, but the 

 intermediate forms link them indissolubly by a marvellously 

 gradual series of transitions. The same fact is well illus- 

 trated by another series of fresh water snails (Paludind), and 

 not less strikingly among those extinct Cuttlefishes which 

 are known as Ammonites, and have perfectly preserved 

 shells. Similarly, though less perfectly, the modern croco- 

 diles are linked by many intermediate forms to their extinct 



