ANCIENT LIFE ON THE EARTH 23 



however, be considered as pelagic mollusca, and 

 probably as the ancestors of the Cephalopoda. True 

 Cephalopoda appeared in the upper Cambrian but 

 did not attain any importance. Undoubted 

 Gastropoda are represented by conical and spiral 

 shells, all of which are very thin, and probably 

 belonged to pelagic animals. The spiral shell is 

 strongly in favour of these early Gastropoda having 

 been proso-branchiate ; and this agrees with the dis- 

 covery that some of the Opisthobranchs still inherit 

 the twist in the visceral nerve-loop which is charac- 

 teristic of the Prosobranchs. The early Pelecypoda 

 were very minute, none of them being more than a 

 quarter of an inch in length. The shells of all the 

 Mollusca consist chiefly of a horny substance, 

 containing but a small quantity of phosphate of 

 lime, and much less of carbonate of lime ; thus 

 differing from the later shells, which are composed 

 almost entirely of calcic carbonate. 



No satisfactory evidence of plant life has been 

 found in any of the Cambrian rocks, unless Oldhamia 

 and the Lower Cambrian oolitic limestones of South 

 Australia 7 are proofs of the existence of a calcareous 

 algse. Eophyton, which was formerly thought to be 

 a plant, has been shown to be due to the trailing of 

 the oral lobes of a Medusa over soft mud. 



SPECULATIONS ON PRE-ORDOVICIAN LIFE 



The first life was pelagic. Let us now see what 

 these dry facts teach us. In the first place, it is 



7 Pro. Linn. Soc. of N. S. Wales, vol. xxi., pp. 571 and 574 

 (1897). 



