24 Annals of the Transvaal Museum 
If compared with the type of H. afvicanus it will be seen, that the relative 
thickness of the last whorl of this specimen is much greater than that of the 
type. However, in the type the height and thickness of the whorls change 
with the diameter and it is difficult to judge with a changing character. Still 
the last whorl of the type has only a thickness of 9-5 mm. where its height is 
10-5 mm. The present specimen is, therefore, relatively thicker than the type. 
There are other differences. The ribs are much lower and closer together 
and, which is especially remarkable, they disappear before reaching the 
tubercles on the umbilical edge. It is just possible, that the last fact is due to 
bad preservation, although the shell seems to be fairly well preserved. There 
is also a slight difference in the lobe-line. The first auxiliary lobe is much 
deeper and the first lateral saddle, the first auxiliary lobe and the first auxiliary 
saddle are together much broader than in the type. However, these differences 
are of such a nature, that one does not feel convinced, that the present specimen 
belongs to a new species; it is also difficult to unite it with H. africanus. 
Genus PACHYDISCUS Zittel. 
Pachydiscus antecursor n.sp. PI. V, figs, i, 2, text-fig. 13. 
Shell discoidal. Whorls increasing slowly in size. Thickness of whorls equal 
to or slightly less than the height. Greatest thickness near the middle of the 
flanks. Sides of whorl and external surface convex; there is no demarcation 
between the external surface and the flanks. The flank curves gently into the 
fairly steep umbilical surface. Height of the umbilical surface moderate. The 
involution is small, being only slightly more than one-third and less than two- 
fifths of the preceding whorl. 
The ornamentation consists of broad, low ribs of different length. The 
longer ribs start near the umbilical surface. Some of them are here slightly 
narrower and higher than elsewhere. They pass over the external surface, 
which is slightly weathered, so that it cannot be made out with certainty, 
whether their strength diminishes on this surface or not. There are indications 
that these ribs are slightly weaker on the external surface. The long ribs are 
irregularly placed. Sometimes a long rib is immediately followed by another 
long rib. Sometimes there is one short rib between two long ones, and in other 
instances there are two or three short ribs between two long ones. The short 
ribs start at irregular distances from the umbihcal surface, but as far as can 
be made out, they all start between the umbihcal surface and the middle of 
the flanks. Their origin is very obscure, for they rise very slowly out of the 
general surface of the shell. Their external end is sometimes as high as that 
of the long ribs, but more often their height is less. The interspaces between 
the ribs are broader than these. The umbilical surface is covered with growth- 
fines, which run from the suture backwards and pass on to the flanks in a 
direction parallel to the ribs. They cannot be traced on the flanks for the 
delicate surface has not been retained there. In passing on to the flanks they 
are concave forwards. The general direction of the ribs is radial. Their internal 
end is slightly convex forwards. External to the middle of the flank the ribs 
are slightly concave forwards. In passing over the external surface they are 
again convex forwards. At least three-quarters of the last whorl is body- 
chamber. Old mouth-edges are difficult to identify. The ribbing of the older 
whorls is relatively somewhat stronger than that of the end of the last whorl. 
The lobe-line is typical for Pachydiscus. Through the nature of its preserva- 
tion it was difficult to develop the lobe-line and much of its fineness has been 
eaten away by the acid. The external lobe is only very slightly shorter than 
