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.) Tunic Strengthened by a Shell. 



GENUS LIMACINA. 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 HYALEA. 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 



