MOLLUSCA. 141 
All the animals of this class inhabit the sea. Some, as the 
Clio and Limacina, frequent the arctic regions, and afford 
the whale a great part of its sustenance. None of the spe- 
cies have hitherto been detected in the British seas. 
M. Cuvier divides the animals of this class into cephalous 
and acephalous. In the latter division he places the genus 
hyalea. The head of the animal of this genus, with its in- 
ferior neck, may, however, be sufficiently recognised to re- 
move all doubt of its existence. 
The characters which may be employed in the classifica- 
tion of this group are numerous ; but the influence which 
their different forms exercise on the habits of the species is 
still unknown. The following disposition of the genera, 
though it has no claims to a natural division, may be useful 
to the student in his investigations. 
(1.) Tunie Strengthened by a Shell. 
Genus Limacrina.—Posterior extremity of the body co- 
vered by a spiral shell. 
The shell, which is very tender, makes one turn and a 
half, is flat on one side, with a large pillar cavity on the 
other. The fins are two in number, one on each side of the 
neck. When the animal swims, the head with the fins are 
protruded. 
This genus was instituted by Cuvier, for the reception of 
the Clio helicina of Captain Phipps, or Argonauta arctica 
of Fabricius. According to Mr. Scoresby, it is found in 
great quantities near the coast of Spitzbergen. 
Genus Hyatea.—Posterior extremity of the body pro- 
tected by two connected shelly valves. 
In the animals of this genus, the body is lodged between 
two plates or valves, united at the base, where they inclose 
the caudal extremity. The ventral valve is nearly flat, with 
