114 HELICID^. 



described. The aperture of the shell being upwards, and 

 the collar of the animal having been brought to a level 

 with it, a quantity of gelatinous matter is thrown out, 

 which covers it. The pulmonary orifice is then opened, 

 and a portion of the air within suddenly ejected, with 

 such force as to separate the viscid matter from the collar 

 and to project it, like a bubble of air, from the aperture. 

 The animal then quickly withdraws further into the shell, 

 and the pressure of the external air forces back the vesi- 

 cle to a level with the aperture, when it hardens and forms 

 the epiphragm. In some of the European species in 

 which the gelatinous secretion contains more carbonate 

 of lime than ours, solidification seems to take place at 

 the moment when the air is expelled, and the epiphragm 

 in these is strongly convex. 



The prevalent characters of this, and probably of other 

 species in a given locality, seem to undergo a considerable 

 change from time to time. When I first visited Salt 

 Island, where this species abounds, ten years ago, it was 

 impossible to find a single specimen with either lines or 

 bands. One uniform color prevailed throughout. At 

 the present time, the banded varieties are said not to be 

 uncommon. [ They have recently been discovered by 

 Dr. Samuel Cabot, in great numbers, on House Island, 

 another of the little islets in the vicinity of Cape Aim, 

 where all of them are of the banded variety. On the 

 Outer Gooseberry, another neighboring islet, he found 

 still another variety. — g.] 



