TESTACEA. 37 



rently ; not quite opaque, but of sensible tbickness and consistence. In 

 protruding, it appears first as a border around tbe orifice, tben slowly and 

 gradually broadening until closing above by meeting of the spreading 

 edges. 



The cloak is completely withdrawn to concealment with the retreat 

 of the tenant of the shell. 



I have not been able to determine tliis animal's food with any cer- 

 tainty. It is often in active motion, and always seeks the highest part 

 of its vessel. Observing to renew the water frequently, it survives I'a 

 considerable time. When much weakened, it retreats within, leaving the 

 rim occupied by the cloak. Numerous empty shells occur in the Scotish 

 seas, but very few with the inhabitant alive. 



Plate VII. 



Fig. 1 .5. Ci/prwa Europwa, shell ; animal in retreat ; cloak diffusing. 



16. Animal in motion, seen from above. 



17. Under surface. 

 IS. Empty slieli. 



Natica alderi.— Plate V., Figs. 9, 10, 11, 12. 



Diameter of the shell under half an inch ; the first volutes the 

 largest, with a deep umbilicus. Colour whitish, speckled brown ; surface 

 very beautiful and delicate, of porcelain aspect. 



The animal inhabiting it is of the nature of a Limax, disproportion- 

 ately large compared with its dwelling. It extends eleven lines. Head 

 truncate, in front thick, with a duplication of the skin towards each side 

 on the upper surface of the anterior part. Two tentacula, like white 

 thorns, one-eighth of an inch long, rise from the portion protruding, juHt 

 at the edge of the shell. The anterior part of the animal is commonly 

 smaller and narrower than the posterior, but less so in some specimens. 

 The whole body is thin, flexible, and much resembling that of a large 

 Planaria ; yet more fleshy, and of a transparent bluish colour. The sole, 

 in its whole extent, is one uniform portion, over part of which the head 

 and tentacula project. The motion of the animal is equal, swift, and 



