ABSORPTION OF THE SHELL. 435 



the absorption has taken place on the outer side, as is proved 

 by the surface shelving down to form an acute edge on its 

 inner side. 



A similar absorption may be observed in the inner whorls 

 of the Harpa articulata ; but it is in this case confined to 

 the central part of the septa, and all the coats are partially 

 dissolved, so as to leave a slit between the cavities of the 

 different whorls. An absorption of the upper septum also 

 takes place in some of the Neritinae, as the Neritina fluvia- 

 tilis ; and it was on a character derived from this circum- 

 stance that M. de Montfort established the latter shell as a 

 genus under the name of Theodoxus. 



These facts distinctly show, that as the animal enlarges 

 the mouth of its shell, it absorbs in a greater or less degree 

 the substance of the inner whorls. This process of absorp- 

 tion, besides furnishing the animal with calcareous matter 

 towards the enlargement of its shell, gives more space for 

 the lodgment of the body, and renders the shell far lighter 

 to carry ; and these advantages are gained without in the 

 slightest degree detracting from its strength, as the outer 

 whorl and spire, which are alone exposed in shells of this 

 form, remain at least as thick as in most other shells. 



In many other univalves the animal, before depositing the 

 laminae which form the inner part of the mouth, absorbs the 

 outer layer of the penultimate whorl, as is evidenced by 

 the ridge with which that part is often surrounded. This 

 is particularly the case in the various species of Turbines, 

 as in Turbo coronatus, T. smaragdus, T. sarmaticus, and in 

 some Fusi, as Fusus despectus. But it is most distinct in 

 the Purpuras, where the Lamarckian character of the genus 

 depends upon this absorption, which causes the concave flat- 

 ness of the inner lip. In the Murices, and other shells 

 which have spines or branching appendages on the front of 

 the whorls, the site of which appendages the succeeding 

 whorls must overlap, these processes are generally absorbed 

 by the animal before it produces the inner lip over their 

 base, as their length would otherwise offer an obstacle to the 

 regular progress of the shell. This absorption of the outer 

 part of the last whorl but one, and of the spines, is evidently 

 effected by the edge of the mantle. In specimens taken 

 while the process is going on there may be observed a notch, 

 formed by this means, in the base of the spines or processes, 

 the completion of which causes them eventually to separate 

 from the shell. A similar effect is produced on a new spe- 

 cies of Sun Trochus, Imperator guilfordiae, where the keels 

 of the whorls are furnished with a central series of spines, 



FF2 



