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. impressuSj 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 difierence 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 Ocellariae, 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 afibrds 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 cylinder or an 

 almost flat 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 quartei* 

 of an inch. 



t In the specimen figured on PI. VI T. vol. xx. fig. 7. the folds are un- 

 usually narrow, but the quincuncial character is well-displayed. 



