2(»2 
PALLIAL OUTGROWTHS. 
and the asynniietry of their internal organs and external orifices can 
only he satisfactorily explained as having been inherited from ancestors 
with spirally twisted intestinal sacs and shells. 
In Vitrina we have the first distinct stage in this process of the 
degeneration of the shell hy its enclosure within the pallial folds, 
which project anteriorly in the form of an incipient limacoid shield 
and laterally as a si)atnliform lobe, both partially overspreading the 
external surface of the shell, which is evidently reduced in size as well 
as in substance, as the body of the animal is now only capable of 
being wholly contained within the shell during dry weather. 
P//>/s(> fontinaliK, though not a Stylommatophore, will serve to show 
a further advance in the develo})ment of the mantle lobes, as although 
they still have a very digitate character, especially ou the left side, 
they have almost oversjiread the shell. The comparatively large size 
of the foot is due to the diminution in size of the shell. 
Pig. 398. Fig. 399. Fig. 4(10. 
Illustrating the stages of the proce.ss leading to the degeneration and loss of the shell owing to 
its enclosure within the pallial lobes. 
Fig. 398. — I'itrina fcUucida (Midi.) X 1.4, Horsforth, near Leeds, showing the first stages of 
pallial e.\pansion. Fig. 'i'Xi.—Physn fimthialis {J..) X 2, River Tome, Doncaster, illustrating a 
further advance of the process. _ Fig. 400. — Atuphipff'lca glutinosa (Midi.), Skidby Drain, Hull, 
collected by Mr. F. W. Fierke, in which the shell is almost entirely enveloped by the m.antle. 
Amphqn'pkd glutinosa, another Basommatophore, idthough able 
to entirely cover the shell by its extended p;dlial lobes, does not 
usually do so, a small rhomboidal dorsal space being generally left 
uncovered, enabling the maculate body of the .uiimal to be seen 
through the transparent shell. The foot does not exhibit any disiiro- 
portionate size in comparison with the size of the shell, owing to the 
degeneration the shell is undergoing having more especially affected 
its substance and not its magnitude. In :dl these cases it is instruc- 
tive to observe the noticeable paucity in the number of whorls of the 
shell and the exceeding delicacy and tenuity of its substance, both 
characters induced by the overwrapping of the mantle lobes. 
The genus Avion illustrates the disappearance of a definite shell, 
the anterior mantle or shield, as it is called, having assumed a very 
tough and leathery consistency, its margins comi)letely overlapping 
and fusing together to f(.)rm a sac enclosing the calcareous granula- 
tions which rejiresent the vestigial shell. 
