200 INVERTEBRATA OF MASSACHUSETTS. 



at the inner base of the tentacula ; foot about half the length 

 and width of the shell, rounded behind, two-lobed in front, and 

 transversely divided at the anterior third. Respiratory orifice far 

 back, on the right side. 



This little shell is very readily distinguished from any other found 

 on our coast, and seems to be another of the connecting links between 

 the land and water shells, or rather, between those which breathe air, 

 and those which breathe water. It has, accordingly, been frequenUy 

 transferred from one genus to another, according to the conjectures 

 of diflerent writers. I have arranged it where Guilding placed it 

 rather doubtfully, after a series of experiments as to the nature of its 

 respiration. Its habits certainly associate it with the preceding species. 

 It is widely distributed over the seas, and is doubtless conveyed to 

 great distances on floating pieces of decaying timber. 



It varies much in its characters according to its age. Its color 

 varies from light horn-color to deep violet, and sometimes it becomes 

 opake-white. There is usually a single revolving line of rigid hairs 

 just before the suture. When young, it is proportionally broader, and 

 the lower whorl proportionally longer, has but two teeth, and the mar- 

 gin of the lip is not reflexed. A third, and sometimes a fourth, tooth 

 appears at maturity, and also some teeth or tubercles within the right 

 lip. This is, indeed, made a part of its character by European writers ; 

 but, of several specimens sent me by Mr. Sowerby, only one bad them. 

 In fact it must be very doubtful whether the species described under 

 the names of hidentafa, tripUcata, 2^"sillus, aJha, rhigens^ reflexa, and 

 perhaps Firmtni, are any thing more than modifications by age, acci- 

 dent, and locality, of this same species. 



Family LIM^TEJl^N'A, Lam. 



Shell spiral, generally smooth exter7ially, and having the outer margin of 

 the aperture always sharp, and not reflexed. 



Genus PLANORBIS, Lam. 



/S/ie/Z discoidal, whorls apparent above and below, aperture 

 crescent-shaped, remote from the axis of the shell ; opetxulum 

 wanting ; animal with thread-like tentacula. 



