inhabiting the United States. 387 
This shell is peculiar for the complicated form of 
its aperture, which bears:a considerable resemblance 
to the human ear. In mature individuals the lip is 
very much projected, the folds within the aperture 
are prominent, and the aperture greatly contracted. 
The laminated folds within are marked by corre- 
sponding depressions externally, behind the reflection 
of the lip. At different periods of growth the aper- 
ture differs very much.in appearance, and has led 
naturalists into error. When the lip is just begin- 
ning. to be formed, and as yet projects but little, 
there are two projecting teeth on its inner edge 
with a deep sulcus between dent; as these con- 
appearance of lamellar folds, the lower one of which, 
when viewed on a line perpendicular. to the base of 
the shell, hides the other. 'The columellar fold, at 
the same early period, appears more like an inde- 
pendent tooth, to each extremity of which the lip is 
connected. It is this variety which Mr. Say de- 
scribed as a distinet species, under the name of Porx- 
eyra avira. This opinion [I derive not so much 
from his descriptions as from the examination of 
original specimens, collected and labelled by him, 
now in my possession. I also consider Hetax Fera- 
siàna, Monicawp, (Mém: de la Soc. de Physique 
et d' Hist. Nat. de Genève, VI. 538. Plow, fig. 2,) 
to represent this shell. The specific. description of 
this author would apply to several of our tridentate 
Species, but the engraved figure represents: very 
exactly a variety of this species with an immature 
aperture. M. Férussac referred H. Texasiana to 
