46 
THE SILURIAN BEACH. 
reefs and islands now. The modern Coral ani- 
mals are chiefly Polyps, but the prevailing Corals 
of the Silurian age were Acalephian Hydroids, 
animals which indeed resemble Polyps in certain 
external features, and have been mistaken for 
them, but which are nevertheless Acalephs by 
their internal structure. In these Corals the 
body, instead of being divided into chambers by 
the vertical partitions so characteristic of the 
Polyps, is divided at regular distances by hori- 
zontal floors. I subjoin a 
wood-cut of a Silurian Coral, 
which does not, however, show 
the peculiar internal structure, 
but gives some idea of the gen- 
eral appearance of the old Hy- 
droid Corals. We have but 
one Acalephian Coral now living, the Millepore ; 
and it was by comparing that with these ancient 
ones that I first detected their relation to the Aca- 
lephs. For the true Acalephs or Jelly-Fishes we 
shall look in vain ; but the presence of the Aca- 
lephian Corals establishes the existence of the 
type, and we cannot expect to find those kinds 
preserved which are wholly destitute of hard 
parts. I do not attempt any description of the 
Polyps proper, because the early Corals of that 
class are comparatively few, and do not present 
features sufficiently characteristic to attract the 
notice of the casual observer. 
