THE NAUTILUS. 
7 
“The species enumerated below have been divided by authors 
into two sections, Triodopsis and Mesodon, but such division seems 
to be artificial. Some species of Triodopsis are known to have 
varieties lacking lip teeth, and these would technically fall into 
Mesodon. 
“ In other cases, such as the group of Idaho and Washington spe¬ 
cies, all the transitions from tridentate to toothless apertures occur. 
The group of P. appressa is also a transition group. Tryon has 
resuscitated the section names Xolotrema and Ulostoma. The first 
of these is a Rafinesquian name totally unidentifiable; the second 
was proposed by Albers for species of Polygyra ss. and Triodopsis ss., 
and did not include either of the forms Tryon uses the name for! 
Aplodon Raf. has also been used in this connection ; it is positively 
unidentifiable.” Whoever has had occasion to review the points 
referred to by Mr. Pilsbry, cannot fail to indorse his conclusions. 
Triodopsis as a name is as unsatisfactory as Mesodon ; but there it 
is, and it will have to stick. Xolotrema and Ulostoma must be con¬ 
signed to the waste basket; fortunately they are book names rather 
than names in practical use. 
Touching the matter of distribution in connection with the fore¬ 
going, the farthest outpost of Triodopsis -f Mesodon in the Southwest 
is that occupied by T. levettei at Fort Huachuca and Tucson, Ari¬ 
zona, about on the line 32° N. latitude and 111° longitude W. 
While Santa Fe, New Mexico, where the type described by Bland 
was collected, is considerably further to the north and east. At the 
first named place the three genera, Arionta, Patula and Triodopsis 
occur in close geographical proximity, but there are wide gaps 
between these localities and the west coast, and between the same 
localities and the nearest eastern localities of Triodoj)sis-\- Mesodon. 
So the more northerly line of distribution of TriodopsisMesodon is 
interrupted or discontinuous as I have pointed out elsewhere, 2 along 
the line as we proceed eastward from the Pacific Coast through the 
states adjoining the southerly boundary line of the Dominion of 
Canada, i. e. Washington, northern Idaho and Montana (according 
to Binney) ; then comes the gap between western Montana and the 
eastward. 
Within the States above named w T e find Triodop>sis-\~M es °d° n > 
Patula and Arionta in geographical proximity as in the southerly 
region previously named. We find the Triodopsisp Mesodon 
2 Proc. U. S. Nat. Mus., Yol. XYI, pp. 749 — 50. 
