THE CAMBRIAN PERIOD. 83 



actual worms themselves in the fossil condition, but we have, 

 nevertheless, abundant traces of their existence. In some 

 cases we find vertical burrows of greater or less depth, often 

 expanded towards their apertures, in which the worm must 

 have actually lived (fig. 30), as various species do at the pres- 



Fig. 30. Annelide-burrows (Scolitfiua linearis), from the Potsdam Sandstone 

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



ent day. In these cases, the tube must have been rendered 

 more or less permanent by receiving a coating of mucus, or 

 perhaps a genuine membraneous secretion, from the body of 

 the animal, and it may be found quite empty, or occupied by 

 a cast of sand or mud. Of this nature are the burrows which 

 have been described under the names of Scolithus and Scoleco- 

 derma, and probably the Histioderma of the Lower Cambrian 

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

 worm seems to have inhabited a double burrow, shaped like 

 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- 



