ON A PIECE OF CHALK 85 



placed side by side, we are led from one to the other by the 

 most gradual progress, if we follow the course of Nature 

 through the whole series of those relics of her operations 

 which she has left behind. It is by the population of the 

 chalk sea that the ancient and the modern inhabitants of 

 the world are most completely connected. The groups 

 which are dying out flourish, side by side, with the groups 

 which are now the dominant forms of life. Thus the chalk 

 contains remains of those strange flying and swimming 

 reptiles, the pterodactyl, the ichthyosaurus, and the plesio- 

 saurus, which are found in no later deposits, but abounded 

 in preceding ages. The chambered shells called ammonites 

 and belemnites, which are so characteristic of the period 

 preceding the cretaceous, in like manner die with it. 



But, amongst these fading remainders of a previous state 

 of things, are some very modern forms of life, looking like 

 Yankee pedlars among a tribe of Red Indians. Crocodiles 

 of modern type appear; bony fishes, many of them very 

 similar to existing species, almost supplant the forms of 

 fish which predominate in more ancient seas; and many 

 kinds of b'ving shell-fish first become known to us in the 

 chalk. The vegetation acquires a modern aspect. A few 

 living animals are not even distinguishable as species, from 

 those which existed at that remote epoch. The Globigerina 

 of the present day, for example, is not different specifically 

 from that of the chalk; and the same may be said of many 

 other Foraminijera. I think it probable that critical and 

 unprejudiced examination will show that more than one 

 species of much higher animals have had a similar longevity; 

 but the only example which I can at present give confidently 

 is the snake 's-head lamp-shell (Terebratulina caput serpen- 

 tis), which lives in our English seas and abounded (as 

 Terebratulina striata of authors) in the chalk. 



