208 Mr. Toulmin Smith on the Classification 
cuncial figure : moveable processes conspicuous : wall often 
thick. 
In this, as in the former species, must be included forms which, 
at first sight, vary to a considerable extent, but which I conceive 
to mark either distinct varieties or distinct ages. It is however 
almost impossible, in specimens in a good state of preservation, 
to mistake the species, the characteristic type of fold being found 
in all with little variation. The wall is usually much thicker 
than in V. impressus, and the fold begins from the base. In some 
specimens the fold is so deep that a thickness of nearly two lines, 
— sometimes even more *, — is attained in the wall of the pouch. 
The folds are almost equally broad down their whole depth, so 
that in this species the general aspect, both external and in- 
ternal, is level, contrary to what takes place in V. muricatus, and 
which difference will best be understood by comparing figs. II 
and 12 of PI. XIII. which represent sections of these two species f. 
It occasionally happens that the outer surface is not quite so 
regularly marked as the inner, though still each depression on 
one side is accompanied by a corresponding elevation on the 
other. This variance is probably owing to some circumstance 
connected with the process of the fossilization of the individual. 
Hence even a partial angularity in the margin of the folds is, in 
some such specimens, seen on the outer surface of the wall of 
the pouch. Such specimens appear to have been partly crushed 
before or during fossilization. The inside preserves all the usual 
regularity of the fold. 
It occasionally, though rarely, happens that the quincuncial 
figure is not maintained ; in all specimens there must necessarily 
be places where it is not truly kept, owing to the mode of increase 
of the animal from base to margin. 
The so-called Ocellarmj so often mentioned, belong to this 
species. What are thus named are either young specimens, or 
the lower parts of full-sized ones. These are usually nearly cy- 
lindrical at the upper part and taper very gradually to the base ; 
or form, altogether, a very acutely pointed inverted cone. 
There is no species which affords more entire disproof of the 
contractile theory than this. It is obviously impossible that any 
membrane should contract in the way in which this form exists, 
— and with no variation whether forming a close eylinder or an 
almost fiat disc. 
I am well-aware that this is the species to which appeal will 
always be made in opposition to the views which have been stated 
* I have an instance in which the thickness of the wall exceeds a quarter 
of an inch. 
t In the specimen figured on PI. VII. vol. xx. fig. 7. the folds are un- 
usually narrow, but the quincuncial character is well-displayed. 
