32 EXTINCT MONSTERS 



age, and have been figured under the names Scolithus, Histioderma, 

 and Arenicolites. They are nearly straight, and descend vertically 

 through the rock. Such are abundant in that ancient formation 

 the Potsdam Sandstone of North America ; in the Clinton forma- 

 tion, also of that country ; and in the Stiperstones of Shropshire. 

 Even in the Pre-Canibrian rocks of the west of Sutherlandshire 

 there have been discovered of late years some long dark lines 

 which are believed to be the burrows of marine worms pulled out 

 to great lengths by the " shearing," or pulling-out process to 

 which these rocks have been long ago subjected. For the full 

 and complete interpretation of many of the curious markings 

 known to geologists, a more accurate knowledge of the markings 

 made by living animals will doubtless be necessary. 



Various worms of the present day, such as the common lug- 

 worm, are known to form long, wandering, tortuous channels in 

 the sand of the seashore, a little distance below the surface. 

 These worms feed on particles of organic matter scattered through 

 the sand or mud, through which it eats its way. Such burrows 

 cross and intersect each other in various ways, and as the worm 

 proceeds on its course, they become filled up in the rear by the 

 sand which has passed through its body. This is how worm-casts 

 seen on the seashore at the present day are made. It appears, 

 in the light of more recent researches, that many markings found 

 in some of the more ancient (Palaeozoic) rocks, and which have 

 been formerly described as "fucoids," i.e. seaweeds, under such 

 names as Palceochorda, are in reality the filled-up burrows of 

 marine worms. These have now been re-christened Planolites. 

 But there are some who consider them to be tracks made on the 

 surface, not burrows. 



A great many true trails, or tracks of worms, etc., that is 

 markings made by the animal dragging its soft body over the 



