168 LAND AND FRESHWATER 
marks the commencement of the formation of the columellar tooth. 
Behind the constriction the inside of the whorl appears thicker, and 
is much more polished; with the constriction this contracts, leaves 
the outer surface of the shell, and continues as a rim, like the sharp 
thread of a screw, running down and round the columellar margin 
of the peristome in the more or less blunt tooth-like process, cha- 
racteristic of the genus. Situated also at the constriction on the 
roof of the whorl at this point may be seen a long tube-like ridge, 
very similar to the external tube of Alyceus, only that it diminishes 
from the back forwards. The position of the operculum, as regards 
both this and the lower rim, is at the back. It does not seem to 
me at all clear for what purposes this internal formation has been 
created. Possibly the extremity of the foot carrying the operculum 
travels along the screw-like thread, and the ridge above may give 
the necessary guiding surface to the operculum when the animal 
issues from the shell. The operculum, situated as it is so far from 
the aperture, would require some fulcrum ox guiding-edges to pass 
it evenly and smoothly out of the shell.” 
On Plate L., in figures 7, 7a, and 8, 8a, I have endeavoured to 
show the position of the operculum and constriction, from the front, 
of D. pachycheilus and D. blanfordiana: where t represents the 
spiral rim; c, the position of the constriction ; 7, the upper ridge or 
tube. 
Figs. 75 and 7e¢ are respectively a side view and plan of the 
relative positions of the operculum and the commencement of the 
spiral rim. 
The peculiar characteristics of Diplommatina are :—1. The very 
marked constriction of the penultimate whorl, situated generally 
above the aperture, in some species behind it, more or less distant. 
2. In the short internal parietal rib just at the beginning of the 
last whorl, and in the twisted columella which terminates in the 
aperture with a tooth, sometimes placed so far internally as to be 
hardly visible, but very rarely becoming nearly obsolete. 3. The 
whorls are more or less transversely costulated, occasionally this 
is absent on the last two*. The first and last characters assimilate 
it to Alyceus, and the short internal tube or rib has also some re- 
semblance, lying as it does close behind the constricted portion of 
the whorl. Compared with <Alycwus, even with animals of the 
larger species, the eyes are comparatively much larger, and the 
tentacles are longer and more slender. It has not the burrowing 
habit of this genus, which perhaps accounts for this very marked 
difference of their development. 
The teeth of the radula are generally so crowded together 
(Plate L. fig. 6a), that only the central and the next lateral can 
usually be seen. However, on dissecting out several lingual ribbons, 
it occasionally happens that they become flattened out and separated 
(fig. 6). For a typical example of the radula and operculum in 
this genus I have selected the largest species as yet known, D. in- 
* Species are found both dextral and sinistral over the whole range of the 
distribution of the genus. 
