Annals of the Transvaal Museum 
39 
The external surface of the shell is ornamented by a continuous keel. This 
is bordered on both sides by a broad, shallow groove. The flanks are covered 
by numerous more or less radial ribs. In one specimen these ribs are slightly 
concave forwards over their whole length and on every part of the shell. In 
two other specimens they are fairly straight on the early stages, while on the 
last whorl they become convex forwards on the middle of the flanks, and 
slightly concave forwards on the outer and on the inner portion of the flanks. 
They start at the middle of the umbilical surface in a strongly backward 
direction and they terminate on reaching the grooves on the external surface. 
The ribs are comparatively broad and low; they are highest at the umbilical 
edge and only just visible at their external end. They are separated by very 
broad, shallow interspaces. The ribs are ornamented by a series of tubercles, 
which are placed in five spiral rows over the* shell. The inner row is situated 
at the umbilical edge. The tubercles of this row are high, narrow and radially 
elongated. On the earlier whorls, especially of one specimen where they have 
not suffered from weathering, they are seen to end in a sharp point. On the 
later whorls they lose this extreme acuteness. The second row is situated near 
the middle of the flanks. It is not present in the very early stages of the shell. 
At a diameter of from 12 cm^ to 15 cm. its tubercles are small knobs, which 
are much lower than the tubercles of the first row and which have a radially 
elongated upper edge. On later whorls these tubercles are represented by a 
shght thickening of the ribs and on the end of the last whorl of our largest 
specimen even this thickening has disappeared; however, this portion shows 
signs of old age, the ribs having become irregular and flattened and in places 
even having been reduced to mere wrinkles. The third row is situated external to 
and at some distance from the middle of the flank. In the early stages of the 
shell the involution is such, that the preceding whorl is covered by the following 
one up to the third row of tubercles. The tubercles are then drawn out into 
long spines, which lean against the umbilical surface of the following whorl. 
On the cast these spines are only represented by high rounded tubercles. The 
row of tubercles diverges slightly forwards with the umbilical suture and, there- 
fore, in later stages the row is separated from the umbilical surface of the 
following whorl. The spines become shorter as the separation becomes greater. 
On the last stages of the shell these tubercles have been reduced to a mere 
thickening of the rib, which, however, is even then thicker and higher than the 
corresponding tubercle of the second row. The upper edges of the spines and 
tubercles of this row are spirally elongated. The fourth row of tubercles is 
completely covered up by the later whorls in the young stages of the shell. 
This row is situated close to the third one and it appears as soon as the third 
row of tubercles and the umbilical suture are a short distance apart ; but even 
then they do not appear completely. On the last whorl they appear to be 
thick and fairly high tubercles, with a moderately long, transversely elongated 
upper edge. The fifth row is only visible on the last whorl. It lies slightly 
nearer to the keel than to the fourth row. The fourth row lies practically mid- 
way between the third and the fifth. The tubercles of the fifth row are high, 
radially very narrow and transversely very long. 
The lobe-line of three of our specimens (of the other two specimens they 
were not developed) differs considerably from that given by Baily. It also 
differs from the lobe-line drawn by Woods. The external saddle is large. It is 
cut into two by a secondary lobe, which passes far backwards in one instance, 
but which is very small in another. The two portions are practically equal in 
one, while in the two others the inner portion is smaller than the outer. The 
first lateral saddle is just as high or higher than the external saddle. It is 
