MOLI.USCA. 443 



their different forms exercise on the habits of tlie species is 

 still unknown. The following disposition of the genera, 

 though it has no claims to a natural division, may be use- 

 ful to the student in his investigations. 



(1.) Tunic Strengthened by a Shell. 



6. LiMACiXA. — Posterior extremity of the body covered 

 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 tlie fins 

 are protruded. 



This genus was instituted by Covier, for the reception 

 of the Clio helicina of Captain Phipps, or Arganauta arc- 

 tica of Fabricius. According to Mr Scokesby, it is found 

 in great quantities near the coast of Spitzbergen. 



7. Hyalea. — Posterior extremity of the body protected 

 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 

 an uneven margin, narrow anteriorly, but expanding be- 

 hind, and terminating in three projecting points. From 

 the middle point four ribs diverge forward, and a muscle 

 arises, which, fixed in the superior viscera, enables the ani- 

 mal to withdraw into the shell. The dorsal valve is short- 

 er than the preceding, the margin flat and circular, and the 

 middle convex outwardly. In the space between the late- 

 -ral margin of the two valves, on each side, the branchiae 

 are situate, in a dupHcature of the tunic, the sides of 

 which are furnished with filaments The fleshy neck sup- 

 ports the two membranaceous expansions; between which 

 and the base the mouth is situate, surrounded by two 

 lips, and strengthened within by two {lo>liv cheeks. The 



