THE GEOLOGICAL RECORD 191 



Either there is no record of the missing links in the 

 Secondary formations, or, what is perhaps more probable, 

 the break between the Secondary and Tertiary beds was of 

 such enormous duration as to afford time for the simultaneous 

 dying out of numerous groups of gigantic reptiles and the 

 development in all the large continents of much higher and 

 more varied mammals. This seems to imply that a large 

 portion of all our existing continents was dry land during 

 this vast period of time ; the result being that the skeletons 

 of very few of these unknown forms were fossilised ; or if 

 there were any they have been subsequently destroyed by 

 denudation during the depression and elevation of the land 

 which we know to have occurred. 



We will now consider these great geological periods 

 separately, in order to form some conception of the changes 

 in the world of life which characterised each of them. 



The Primary or Palceozoic Era 



The Palaeozoic differs from the two later eras of 

 geology in having no known beginning. The earliest fossils 

 iare found in the Cambrian rocks ; they consist of a few 

 obscure aquatic plants allied to our Charas and Algae, and 

 some lowly marine animals allied to sponges, crinoids, and 

 annelids. But there are also many forms of shell-bearing 

 Mollusca, which had already developed into the four great 

 classes, lamellibranchs, pteropods, gasteropods, and cephalo- 

 pods ; while some groups of the highly organised crustaceans 

 were abundant, being represented by water-fleas (ostracods) 

 and numerous large and varied trilobites. Besides these, 

 :he curious Molluscoidea were fairly abundant, Terebratulae 

 low first appear, and, as well as the genus Lingula, have 

 :ontinued to persist through all the subsequent ages to the 

 oresent time. Great masses of rocks stratified and un- 

 ratified exist below the Cambrian, but have mostly been 

 netamorphosed by internal heat and pressure, and contain 

 10 recognisable organic remains. 



Geologists have been greatly impressed by this sudden 



ppearance of marine life in such varied forms and com- 

 paratively high organisation, and have concluded that the 



tratified formations below the Cambrian must probably have 



