CRETACEOUS PERIOD iii 



by force of prolificness, they became true rock-builders ; for 

 the chalk is largely composed of their shelly coverings. It 

 has been estimated that one cubic inch of chalk represents 

 the remains of more than a million minute organisms, mostly 

 foraminifers. 



Along the coasts and estuaries birds of various kinds were birds 

 in evidence, water-habits having come greatly into vogue. As 

 the remains found all belong to the latter part of the Period, 

 a wide hiatus occurs in bird history, and great changes cause 

 no surprise. 



Toothed birds, however, continued ; but the tail had 

 undergone important modifications. It did not now consist 

 of a long lizard-like structure decked with feathers. The 

 bone had been considerably shortened ; and the feathers 

 were doubtless so arranged as to spread out more or less fan- 

 wise as in the case of modern birds. Another link, therefore, 

 with reptiles had been broken. Nor were wings, so far as is 

 known, provided with usable claws. 



Birds exhibited still more changes. Some genera, for 

 instance, had been through a series of modifications resulting 

 in the loss of the power of flying. It would seem from this, 

 that in certain regions birds must have found it safe and 

 convenient to abandon, more or less, aerial flight ; and that 

 their habits had been confirmed by successive generations. 

 In this way, through long disuse, a posterity had in time 

 appeared entirely deprived of power to fly. These Cretaceous 

 birds had little left to show that their remote ancestors 

 possessed usable wings ; for the bone of the upper part of the 

 fore-limb alone remained to testify to this. They may be 

 said, moreover, to have wellnigh lost their " land legs " ; 

 for their limbs turned outwards from the body and pedes- 

 trianism must have been a laborious proceeding. 



The life of these birds no doubt was spent mostly in the 

 water, as in the case of grebes and divers. Indeed they 

 were, so to speak, compounds of those birds, and pro- 

 bably their ancestral forms. They had doubtless become 

 highly expert at catching fish, for their advanced speciali- 

 sation for aquatic life points to long acquaintance with 

 the art. 



