THE SILURIAN BEACH. 
51 
nny liard covering, could hardly be preserved ; 
hut, like the marine Worms of our own times, 
they were in the habit of constructing envel- 
opes for themselves, built of sand, or some- 
times from a secretion of their own bodies, and 
these cases we find in the earliest deposits, giving 
us assurance that the Worms were represented 
there. I should add, however, that many im- 
pressions described as produced by Worms are 
more likely to have been the tracks of Crustacea. 
But by far the most characteristic class of Ar- 
ticulates in ancient times were the Crustaceans. 
The Trilobites stand in the same relation to the 
modern Crustacea as the Crinoids do to the mod- 
ern Echinoderms. They were then the sole rep- 
resentatives of the class, and the variety and rich- 
ness of the type are most extraordinary. They 
were of nearly equal breadth for the whole 
length of the body, and rounded at the two 
ends, so as to form an oval outline. To give any 
adequate idea of the number and variety of spe- 
cies would fill a volume, but I mav enumerate 
some of the more striking differences : as, for in- 
stance, the greater or less prominence of the an- 
terior shield, — the preponderance of the poste- 
rior end in some, while in others the two ends 
are nearly equal, — the presence or absence of 
prongs on the shield and of spines along the 
sides of the body, — appendages on the head in 
