22 
Annals of the Transvaal Museum 
broad and convex and provided with a narrow, low, cork-like keel. The con- 
vexity of the shell is greatest between the external surface and the flanks, but 
there is no sharp line of demarcation between the two surfaces. The umbilicus 
is extremely narrow. The flank slopes gradually inwards towards the umbilical 
surface, which is very low and strongly convex. The inner portion of the 
umbilical surface slopes away from the centre and towards the plane of 
S57mmetry, and therefore the radius of the umbilical suture is longer than that 
of the middle of the umbilical surface. The involution is such, that only an 
extremely narrow* strip of the preceding whorl remains uncovered. 
The surface of the shell is smooth, except perhaps for growth-lines, especially 
in the vicinity of old mouth-edges. Old mouth-edges are visible as very shallow 
grooves bordered by fine ridges and growth-lines. There are three of them close 
together on the last whorl. Their general direction on the flanks is more or less 
radial. They are slightly concave forwards on the inner portion and slightly 
convex forwards on the middle of the flanks, w*hile they are again concave 
forwards on the external portion of the flanks and on the outer surface. They 
run very far forwards on the outer surface, making an extremely sharp angle 
with the median line. 
The lobe-line is typically that of Desmoceras as represented by D. sugata 
Forbes sp. All the saddles are bifid and all the lobes except the external one 
are trifid. The external lobe is slightly longer than the first lateral lobe. Ten 
Text-fig. 12. Desmoceras compactum. Lobe- 
line of type up to near the umbilical suture 
on a radius of 7 mm. x 3. The long dotted 
line indicates the approximate border be- 
tween the flank and the umbilical surface; 
the short one indicates the shortest radius 
of the latter surface. 
saddles are visible up to the smallest diameter of the umbilical surface; these 
are arranged in a straight line and diminish gradually in height towards the 
umbilicus. 
Measurements : 
Diameter 
i 8-2 mm. 
(i-oo) 
Height of last whorl ... 
10 
( 0 - 55 ) 
Thickness of last whorl 
8-3 
(0-46) 
Height of penultimate whorl . . . 
4-8 „ 
(0-26) 
Thickness of penultimate whorl 
4-9 » 
(0-27) 
Diameter of umbilicus ... 
1-7 .. 
(0-09) 
The present form belongs to the group of Desmoceras sugata Forbes sp. 
Its whorls, however, differ from those of the Indian form in section, firstly 
because the keel is not so high and secondly because the greatest thickness of 
the whorl of the African species lies at the external edge of the flanks. The 
umbilicus is also relatively narrower. The old mouth-edges are also differently 
shaped. In D. compactum they are clearly visible near the umbilicus, but this 
is perhaps not true of all of them. They are, however, slightly convex forwards 
on the middle of the flanks and this is not so in D. sugata. Further they form 
a very sharp point on the external surface, a point which is much sharper than 
in D. sugata. There are slight differences in the lobe-line. The external lobe is 
longer than the first lateral one, not shorter, the saddles do not diminish so 
rapidly in size towards the umbilicus and there may be one or two saddles 
more in D. compactum. Our form differs from D. sugata figured by Yokoyama 
( 9 , PI. XX, fig. ii) in the section of the whorl and in the relatively narrower 
