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- Cambrian 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 Falceochorda, 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 
