THE CAMBRIAN PERIOD. 83 
of Ireland. In other cases, as in Arenicolites (fig. 32, 6), the 
worm seems to have inhabited a double burrow, shaped like 
an ra Te ZEB ————_— 
a ae c ae 
SEZ FE = = = = > 
=e ae S 
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 (A7enzcola) 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 (P%ylopfoda). One of the most characteristic of 
