sively removed, the mature shell has an appearance quite different from that of the immature, 

 being nearly straight and bluntly truncated, instead of well curved and posteriorly acuminate. 



The same individual in its two extreme stages of ijrowth, without the controlling' inter- 

 mediate examples, might be excusably described as distinct species. This probably partly explains 

 \vhy the name of D. Bednalli Pilsbry & Sharp, has been added to that of D. intercalatuiii 

 Gould, the former being an old individual, and the latter a young one. 



One shell, compared in the Xatural History Museum, London, with that labelled D. 

 iiitercalatiiiii Gould, was identical, and represents our immature, curved, sharp-pointed stage. 

 The figure given in Tryon's Man. of Conch. corresponds with it, as does also the description 

 there transcribed, even to the origin of its secondary riblets, first in the two interspaces on the 

 outer curve, and somewhat later in the lateral ones and on the concave side, as italicised in 

 the manual. Hence, though the type locality of this species is given as China Seas, the identity 

 of our South Australian form is indisputable. 



The number of ribs very rarely remains the same throughout the entire of the shell; 

 thus one with eleven rather acute ribs at the posterior end has but eleven at the anterior. 

 Almost always the ribs become more numerous with age. The increase is effected in two ways, 

 by intercalation and by rib splitting. 



1. By intercalation. Generally in the centre of an interspace a riblet arises, and gradually 

 enlarges until it equals the original ribs. The stage of growth at which this begins is variable, 

 not only in different shells, but in the several interspaces of the same shell. For instance, when 

 the individual is quite immature every interspace may bear a riblet, whereas when senile there 

 may be only the first indication of one. Again, one interspace may show a riblet very soon, and 

 later, other interspaces may develojj them at varying distances as in the typical D. inter calatiim. 

 Besides these secondary riblets, eventually tertiary riblets may arise in their interspaces and further 

 multiply the costations. Still another variation is to be seen — instead of a single secondary 

 costula arising in an interspace, two riblets of equal dimensions may develop simultaneously. 

 This twin intercalation alone maj- be found in an example, or there may be every combination 

 of single and of doublé intercalation. 



2. By rib-splitting. A groove begins to form in a rib, and gradually grows in depth and 

 width until it divides it into two. This groove may begin on the centre or on any part of the 

 side of the rib. The ribs so formed may subsequently be cut up by other grooves. Sometimes 

 two or three of such furrows may appear on the same rib at the same time and enlarging at 

 an equal rate form three or four subequal riblets. 



The two methods, intercalation and rib-splitting, may occur alone in respective individuals, 

 or both in the same example, and there may be any conceivable ratio between the two methods 

 in different specimens. 



The contour of the ribs may vary greatly. They may be at their inception narrow and 

 comparatively high, and may so continue throughout their length. Or after some increase in 

 size they may begin to decline in height until they almost fade out and leave the anterior part 

 of the shell nearly smooth. Their shape may completely change; whereas at first, they may 

 have concave interstices so as to resemble a fluted column, the ribs may widen out and become 



