THE CAMBRIAN PERIOD. 83 



of Ireland. In other cases, as in Arenicolites (fig. 32, l>), the 

 worm seems to have inhabited a double burrow, shaped like 



Fig. 30. Annelide-burrows (Scolithus linearis). from the Potsdam Sandstone of 

 Canada, of the natural size. (After Billings.) 



the letter U, and having two openings placed close together 

 on the surface of the stratum. Thousands of these twin- 

 burrows occur in some of the strata of the Longmynd, and it 

 is supposed that the worm used one opening to the burrow as 

 an aperture of entrance, and the other as one of exit. In other 

 cases, again, we find simply the meandering trails caused by 

 the worm dragging its body over the surface of the mud. 

 Markings of this kind are commoner in the Silurian Rocks, 

 and it is generally more or less doubtful whether they may 

 not have been caused by other marine animals, such as shell- 

 fish, whilst some of them have certainly nothing whatever to 

 do with the worms. Lastly, the Cambrian beds often show 

 twining cylindrical bodies, commonly more or less matted 

 together, and not confined to the surfaces of the strata, but 

 passing through them. These have often been regarded as 

 the remains of sea-weeds, but it is more probable that they 

 represent casts of the underground burrows of worms of simi- 

 lar habits to the common lob-worm (Arenicola) of the present 

 day. 



The Articulate animals are numerously represented in the 

 Cambrian deposits, but exclusively by the class of Crustaceans. 

 Some of these are little double-shelled creatures, resembling 

 our living water-fleas (Ostracoda). A few are larger forms, and 

 belong to the same group as the existing brine-shrimps and 

 fairy-shrimps (Phyllopoda). One of the most characteristic of 



