of North American IlelicidcB. 441 



tilled to perform, it is difficult, indeed impossible, in the present 

 state of onr knowledge, to compreliend. When tlie teeth, -from 

 their form and number, contract to a considerable degree the 

 apertnral space through which the animal can alone protrude 

 itself, thev app/ear capable of aflbrding to it protection against 

 the entry of its enemies, or a means of removing particles of 

 earth or other foreign matter adhering to its mucous-covered 

 body, when withdrawing into its habitation. I am not, how- 

 ever, aware of any differences in the habits of the animals of 

 shells Avith and without teeth, which warrant sucli or ati}^ other 

 theories as to their value. Shells with large and small, toothed 

 and toothless apertures, are found inhabiting the same localities, 

 and subject to the same external influences and circumstances. 

 The form and position, and with very trifling exceptions the 

 number of the teeth, are constant and reliable specific charac- 

 ters. In proof of this, and as evidence of their value, I may 

 mention that when the aperture of a toothed shell is accident- 

 ally broken, the animal not only repairs the injured part, but 

 reproduces the teeth. I have specimens oill. appressa^ injlecta, 

 7H07iodon, septemvolva, and tridentata, in which, after the for- 

 mation of the parietal tooth, the aperture and part of the last 

 whorl adjoining were broken off, and the animals not only 

 reconstructed the reflected lips of their shells, but added also 

 the parietal teeth, the old ones remaining as monuments of the 

 destroyed apertures. In one of my specimens of H. triden- 

 tata^ after completion of the shell, the animal continued the 

 last whorl about 2 mill, beyond the lip, partially reflected the 

 new lip, and added two incipient teeth in advance of the 

 old ones. On Plate IV., fig. 20, a sketch is given of the 

 very singular reparation of one of my specimens of H. sep- 

 temvolva. The fracture was behind the aperture, which the 

 animal in consequence abandoned, but it formed a new one by 

 reflecting the outer lip, on the lower part of which it added a 

 small tooth, — it found insufficient space for another parietal 

 tooth behind the old aperture. 



