46 THE STORY OF THJ5 EARTH AND MAN. 



the marks which their jointed bodies and numerous 

 side-bristles leave on the sand and mud may, when 

 buried under succeeding sediments, remain ; and ex- 

 tensive surfaces of very old rocks are marked in this 

 way, either with cylindrical burrows or curious trails 

 with side scratches looking like pinnate leaves. These 

 constitute the genus Crusiana, while others of more 

 ordinary form belong to the genus Arenicolites, so 

 named from the common Arenicola, or lobworm, whose 

 burrows they are supposed to resemble. Markings 

 referable to seaweed also occur in the Primordial rocks, 

 and also some grotesque and almost inexplicable or- 

 ganisms known as Oldhamia, which have been chiefly 

 found in the Primordial of Ireland. One of the most 

 common forms consists of a series of apparently 

 jointed threads disposed in fan-like clusters on a cen- 

 tral stem (Oldhamia antiqua). Another has a wider 

 and simpler fan-like arrangement of filaments. These 

 have been claimed by botanists as algEe, and have been 

 regarded by zoologists as minute Zoophytes, while 

 some more sceptical have supposed that they may be 

 mere inorganic wrinklings of the beds. This last view 

 does not, however, seem tenable. They are, perhaps, 

 the predecessors of the curious Graptolites, which we 

 shall have to represent in the Silurian. 



Singularly enough, Foraminifera, the characteristic 

 fossils of the Laurentian, have been little recognised in 

 the Primordial, nor are there any limestones known 

 so massive as those of the former series. There are, 

 however, a number of remarkable organisms, which 



