532 MR. B. B. WOODWARD ON THE GROWTH AND [JunC 14, 



not merely to the outer lip and columella alone as in normal growth, 

 but all the way round in the plane of the outer hp, as first evinced 

 by the deposition of periostracal layer along and over the outer 

 margin of the enlarged callus (fig. 20), the lines of deposition, or 

 growth, being continuous with those of the outer Hp. In other 

 words, were it not for the overlap of the callus the peristome would 

 be complete, as in Neritina crepidularia and Tomostoma neritoides. 

 Deposition also takes place over the whole surface of the callus. The 

 direction of growth is in this manner completely changed. Instead 

 of developing spirally, round an axis of which the protoconch forms 

 the apex, the shell enlarges radially, the new axis being the pillar- 

 like margin of the septum and its apex the point, on the exterior of 

 the body-whorl, situated immediately over the junction of these two. 

 In this new condition of affairs the callus, which is at right angles 

 to the new axis, lies of course completely athwart the direction of 

 growth and decidedly in tlie way of further extension, so that the 

 animal must have found itself much in the same predicament that a 

 limpet would do were it to be suddenly half-decked when its period 

 of growth was still far from complete. 



A grave problem in its domestic architecture was thus raised, and 

 the solution forms the most interesting feature in the hfe-history of 

 this species ; for layer by layer, as deposition of fresh shelly matter 

 took place without, a corresponding amount of material was removed 

 on the inner side of the callus, and the additional room required 

 thus obtained. Put in homely phraseology, this mode of enlarging 

 a tenement reminds one of nothing so much as of the Irishman, who 

 raised his roof by digging out the floor of his cabin. 



The ultimate outcome of this novel mode of increase is that, in the 

 adult Velates, that portion of the shell included between the margin 

 of the outer lip and a line (A B, fig. 21) joining its extremities and 

 passing round and a little below the apex on the further side is 

 normal, whereas the remainder is formed out of callus past and 

 present. This comes out very clearly in the various sections of the 

 shell presently to be described. Of course the walls around the apex 

 which require to be thickened as the shell increases in size, to make 

 them as durable as the rest, are strengthened in the usual way 

 bv the deposition of fresh shelly matter within, so that, in an old 

 shell, what was once the cavity inhabited by the young animal has 

 become solid shell. 



The changes which take place in the external form of the test of 

 Velates, as was to be expected, find their reflection in the intimate 

 structure of the shell itself. An axial section whose plane passes 

 close to and almost parallel with the edge of the columellar lip, but 

 just misses the apex itself, has been made in each of three young 

 shells of different ages, and the sections stained with picro-carmine 

 to bring out the structure more clearly. In the first, a specimen 

 of about 3 whorls (Plate XXXII. fig. 22 a), the shell-wall near the 

 apex shows three readily distinguishable layers : — the outermost, or 

 periostracal layer, the crystaUiue, and the innermost layer, which in 

 this case consists of the material laid down not merely as a lining to 



