Annals of the Transvaal Museum 
2-3 
umbilicus. Desmocevas obscurus Schliiter sp. (6, p. 70, PI. XXII, figs. 9, 10) 
has a different whorl section and a much wider umbilicus. 
Only one specimen of this species has been found. 
Holcodiscus africanus v. Hoepen. 
1920. Holcodiscus africanus v. Hoepen, 20 , p. 146, PI. XXVI, figs. 3 — 5. 
Three specimens have been found, which are referable to the above species. 
The largest of the three belongs here without any doubt. The specimen is 
fragmentary and most of its shell has exfoliated. A small portion of the shell 
of the last whorl has, however, been preserved and this small portion is re- 
markable. Its ribs are very strong, high, narrow and wire-like. In two places 
this shell-portion passes over the external surface on to the other flank and it 
appears that the ribs in these two places pass with undiminished strength over 
the external surface. In between, where the shell is broken away, the cast 
shows a siphuncular groove, but not a trace of ribs. In front of the preserved 
shell-portion the cast shows the siphuncular groove and ribs on the flanks, 
which disappear on the external surface. Further forwards the siphuncular 
groove disappears under thin layers of shell and here the strong ribs of the 
flanks pass over the external surface, but very much flattened. In a young 
stage of this shell, which has not suffered from weathering and of which a 
portion of the external surface is visible, the ribs do not pass over this surface. 
The ribs of the penultimate whorl have not suffered from weathering and are 
strong on the flanks, while some have disappeared and the remainder are only 
faintly visible on the siphuncular region. The shell of the type is sound. It is 
quite clear that the comparative smoothness of its external surface is not due 
to exfoliation. However, at the beginning of the last whorl, the shell shows 
a remarkable irregularity. The siphuncular line has here been displaced on to 
one of the flanks, where the ribs are cut off abruptly. The ribs of the other 
flank pass over the external surface, and then bend strongly backwards until 
they meet the displaced siphuncular line. Slightly further backwards the ribs 
on the flank which bears the siphuncular line have irregular directions, some 
even being bent backwards. Possibly this irregularity must be ascribed to 
some accidental cause, perhaps a wound. Perhaps also the character of a 
comparatively smooth siphuncular region must be allowed a certain variability. 
In the two other specimens the ribs become very faint on the external surface. 
A character which appears to be variable is the number of ribs on a limited 
surface, i.e. the relative density of the ribbing. One of the new specimens has 
ten ribs on i cm. peripheral length at a longest diameter of 27 mm. In the 
type there are eight ribs on the same length at the same diameter. This 
difference gives the ribbing of the new specimen a decidedly finer appearance. 
Again, a small specimen has ten ribs on i cm. peripheral length at a diameter 
of about 16 mm. and therefore its ribbing is relatively coarser. 
Holcodiscus africanus? v. Hoepen. 
There is one specimen of a Holcodiscus which resembles very much Holco- 
discus africanus, but which differs from this species in several respects. Its 
dimensions are as follows: 
Diameter 
25-9 mm. (i-oo) 
10-5 „ (0-41) 
lo-i „ (0-39) 
5-4 M ( 0 - 21 ) 
6 „ (0-23) 
8-2 „ (0-31) 
Height of last whorl . . . 
Thickness of last whorl 
Height of penultimate whorl . . . 
Thickness of penultimate whorl 
Diameter of umbilicus... 
