136 
examples, might be excusably described as distinct species. 
This probably partly explains why the name of D. Bednalli, 
Pilsbry & Sharp, has been added to that of D. intercalatum, 
sould, the former being an old individual, and the latter a 
young one. 
One shell, compared in the Natural History Museum, Lon- 
don, with that labelled D. intercalatum, 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 con- 
cave side, as italicised in the manual. Hence, though the 
type locality of this species is given as China Seas, the iden- 
tity of our South Australian form is indisputable. 
The number of ribs very rarely remains the same through- 
out the entire length of theshell; 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. 
l. By intercalation. Generally in the centre of an inter- 
space 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 indi- 
vidual 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 develop them at varying dis- 
tances as in the typical D. intercalatum. Besides these secon- 
dary riblets, eventually tertiary riblets may arise in their 
interspaces and further multiply the costations. Still ano- 
ther variation is to be soon—instead of a single secondary cos- 
tula arısing in an interspace, two riblets of equal dimen- 
sions may develop simultaneously. This twin intercalation 
alone may be found in an example, or there may be every 
combination of single and double 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. Тһе 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 
rer oi at an equal rate form three or four subequal rib- 
ets. 
