64 CEPHALOPODA. 
Most of the species of the Class are destitute of 
any external covering, but a few are protected by 
a shell. An immense number of fossil species, 
however, belonged to the latter division, known by 
the names of Belemnites, Nummulites, Orbulites, 
Ammonites, &c. The most important existing 
species possessing a shell, are te Pearly Nautilus of 
the tropical seas, and the Paper Nautilus of the Medi- 
terranean. The former of these possesses four gills, 
a peculiarity which distinguishes it from all its 
fellows: Professor Owen, therefore, proposes to 
divide the Class into two Orders ; the one ineluding 
the Pearly Nautilus, called Tetrabranchiata, or four- 
gilled; the other including the rest of the existing 
species, named Dibranchiata, or two-gilled. 
As the animals belonging to the Molluscous 
Division are very numerous, I propose to limit this 
volume to those families which are represented by 
species existing in and around the British Islands, 
contenting myself with an occasional slight notice 
of such foreign kinds as have anything particularly 
interesting in their history. I shall therefore at 
onee proceed to the second of these Orders. 
