THEIR LIFE-FORMS. 193 



dwelling on minutiae, which would be out of place in a 

 popular sketch like the present, we may yet advert to some 

 of the more remarkable features in the life of the second- 

 ary ages, such as the great preponderance of nautilus and 

 cuttle-fish like forms among the mollusca ; the marvellous 

 variety of reptilian life, which has led to the designation 

 " the age of reptiles ; " the first unmistakable appearance 

 of bird-life ; and the occurrence of mammals of the lower 

 or pouch-bearing section. There are other noticeable fea- 

 tures too, such as the disappearance of the plants peculiar 

 to the coal-measures, and their replacement by other tree- 

 ferns, by cycads, zamias, pine-trees, and palms ; the dis- 

 appearance of the graptolites, trilobites, eurypterites, and 

 bone-encased fishes that characterised the older systems; 

 the prevalence of homocercal or equal-lobed-tailed fishes un- 

 known in paleozoic times, and similar peculiarities; but 

 these we must subordinate to the more remarkable features 

 above alluded to. 



The highest order of molluscan life is the Cephalopod, 

 or those which move about by the arm-like feelers that 

 encircle the head. To this order belong the nautilus and 

 cuttle-fish the former possessing an external chambered 

 shell, and the latter having no external shell, but support- 

 ed by an internal bone or osselet. The nautilus is now the 

 only representative of the shelled division, and though a 

 number of genera belong to the shell-less orders, they do 

 not occur in anything like overwhelming numbers. In the 

 secondary seas, however, these cephalopodous mollusca 

 were predominating forms, their shells and internal bones 

 occurring in myriads, and this more especially during the 

 oolitic and cretaceous periods. Everywhere throughout 

 the limestones, shales, and clays, their remains are scattered 

 in profusion the chambered shells, from the size of the 

 smallest coin to the circumference of a carriage-wheel, and 



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