THE PRIMORDIAL, OR CAMBRIAN AGE. 45 



the Primordial to the Carboniferous rocks, but are 

 altogether wanting in the more recent formations and 

 in the modern seas. The Trilobites lived on muddy- 

 bottoms, and their remains are extremely abundant 

 in shaly and slaty beds, though found also in lime- 

 stone and sandstone. In the latter they have left 

 most curious traces of their presence in the trails 

 which they have produced. Some of the most ancient 

 sandstones have their surfaces covered with rows of 

 punctured impressions (Protichnites, first foot-prints), 

 others have strange series of transverse grooves with 

 longitudinal ones at the side {Glimactichnites, ladder 

 foot-prints) ; others are oval burrows, marked with 

 transverse lines and a ridge along the middle {Busich- 

 niteSy wrinkle foot-prints). All of these so nearly 

 resemble the trails and tracks of modern king-crabs 

 that there ' can be little doubt as to their origin. ' 

 Many curious striated grooves and bifid marks, found 

 on the surfaces of Primordial beds, and which have 

 been described as plants, are probably only the marks 

 of the oral organs or feet of these and similar crea- 

 tures, which passed their lives in grubbing for food 

 in the soft, slimy ooze, though they could, no doubt, 

 like the modern king-crabs, swim when necessary. 

 Some still more shrimp-like creatures, Hymenocaris, 

 which are found with them, certainly had this power. 



A lower type of annulose or ringed animal than that 

 of the Trilobites, is that of the worms. These crea- 

 tures cannot be preserved in a fossil state, except in 

 the case of those which inhabit calcareous tubes : but 



