72 Transactions of the Boyal Society of South Africa. 
Observations. — This form of Mactroid-shell is represented by a ' 
single specimen with closed valves, the umbones of which have been 
destroyed, as well as the details of the hinge area. Notwithstanding 
the absence of the actual umbones, a view of the left valve exhibits 
a well-elevated and beautifully rounded umbonal region situated 
almost in the centre of the shell and margined posteriorly by an 
obtuse angulation which terminates at the postero-ventral corner. 
The hinge-line is nearly horizontal and not laterally oblique as in 
most examples of the genus Mactra, so that the general contour 
is quadrate rather than triangular. Among Cretaceous species 
referred to the genus Mactra, and to which the present shell 
may be related, is M. valangiensis of Pictet and Campiche (Mat. 
Pal. Suisse, 1865, pi. 8, fig. 9, p. 127) from the Lower Neo- 
comian of Sainte-Croix, Switzerland. This species was founded 
upon a cast showing both valves in the closed condition, being in 
contour and dimensions curiously like the specimen from Zululand, 
but without actual examples for comparison it is difficult to say with 
accuracy how far they resemble each other. If anything, the Swiss 
shell seems to be rather more oblique at the anterior margin, whilst 
the posterior area is less defined and much narrower, measuring only 
10 mm. across, whereas the African shell shows a width in that 
region of 23 mm. 
Again, Pictet and Eoux's Mactra gaultina from the Gres Verts " 
of Perte-du-Ehone (Desc. Moll. Foss. Gres Verts Geneve, 1852, 
pi. 29, fig. 3, p. 407) differs from the present species in having 
compressed valves, a nearly horizontal ventral margin, and a 
relatively longer axis. The casts from the Cenomanian of Con- 
stantine figured and described by Coquand (Geol. Pal. Constantine, 
1862, pi. vii., figs. 1-4, pp. 190, 191) as Mactra didonis, and M. 
WKBVusi are both too trigonal for confusion with this species, as 
also are two shells described by Forbes from the Southern Indian 
Cretaceous as M. tripartita and Gardium inter sectuni (see Stoliczka, 
Cretaceous Pelecypoda, Southern India, Pal. Indica, 1871, vol. 3, 
pi. 5, figs. 8-11, p. 57). 
It is extremely doubtful whether some of the Cretaceous forms of 
this genus hitherto described, about which the dentition is unknown, 
are really Mactra or not ; it is therefore necessary in the present 
case to query the generic determination. The species is dedicated 
to Professor T. Rupert Jones, F.B.S., in admiration of his early 
researches on the geology of South Africa. 
Locality. — Tributaries of the Manuan Creek. 
