MOLLUSKS 



147 



Fig. 1 07, 



Periwinkles clustering upon a rock at 

 Annisquam, Mass. 



The spire of the shell is blunt, and in old individuals its apex 

 is apt to be broken off. The surface of the shell is covered with 

 seaweed and mud, but 

 Avhen cleaned it is seen 

 to be black, with a shin- 

 ing black interior. This 

 snail drills holes through 

 the shells of other mol- 

 lusks and devours them. 

 It is, however, preyed 

 upon by young hermit 

 crabs, that occupy the 

 shells after devouring 

 the mollusk itself. 



The Sand-Flat Snail, 

 (Ncussatrivittata, Fig. 106), 

 can be distinguished 

 from Nassci ohsoleta by 

 its sharp spire, angular suture and regularly granular surface. The 

 shell is five-eighths of an inch long, and two sharp-pointed fleshy 

 processes arise from the posterior end of the foot giving the impres- 

 sion of a pair of tails. There is a pair of long, 

 slender tentacles on the head, and half way up 

 on the side of each there is an eye. The siphon 

 tube is long and curved upward, while the pro- 

 boscis is even longer and extends forward as 

 a flexible tube. The snail applies this proboscis 

 to the surface of other snail shells, bores through 

 by means of its rasping leeth, and then devours 

 the soft parts of the prey. Xassa trivittata 

 appears to feed upon every species of mollusk through whose shell 

 it is capable of boring, and will readily attack individuals of its 

 own species. 



It is found from eastern Florida to Nova Scotia, and is most 

 abundant upon sand flats in Long Island Sound, and on the Massa- 

 chusetts coast north of Cape Cod. It is also found on muddy or 

 stony bottoms and extends into water about 240 feet deep. When 

 the tide goes out it crawls slowly over the moist sand leaving a tortu- 



Fig. loS; PERIWINKLE. 



From life. 



