48 
THE SILURIAN BEACH. 
Acepliala or Bivalves we shall find in great num- 
bers, but of a very different pattern from the 
Oysters, Clams, and Mussels of recent times. 
The annexed wood-cut represents one of these 
Bracliiopods, which form 
a very characteristic type 
of the Silurian deposits. 
The square cut of the 
upper edge, where the 
two valves meet along the back and are united by 
a hinge, is altogether old-fashioned, and unknown 
among our modern Bivalves. The wood-cut does 
not show the inequality of the two valves, also a 
very characteristic feature of this group, — one 
valve being flat and fitting closely into the other, 
which is more spreading and much fuller. These, 
also, were represented by a great variety of spe- 
cies, and we find them crowded together as closely 
in the ancient rocks as Oysters or Clams or Mus- 
sels on any of our modern shores. Besides these, 
there were the Bryozoa, a small kind of com- 
pound Mollusk allied to the Clams, and very busy 
then in the ancient Coral work. They grew in 
communities, and the separate individuals are so 
minute that a Bryozoan stock looks like some 
delicate moss. They still have their place among 
the Beef-Building Corals, but play an insignifi- 
cant part in comparison with that of their prede- 
cessors. 
