84 
Transactions of the Boyal Society of South Africa. 
Family PLEUEOMYID^. 
Genus PLEUEOMYA, Agassiz, 1845. 
Etudes Critiques sur les Mollusques Fossiles — Monographie des 
Myes, 1845, p. 231. 
Pleukomya africana, E. Etheridge. 
Plate VIII., figs. 1-3. 
Myo2)sis (?) africana, E. Etheridge, in W. Anderson, Third Eeport 
Geol. Surv. Natal and Zululand, 1907, pi. 1, figs. 9, 10, p. 81. 
Observations, — It is interesting to be able to recognise in the 
present collection a species which has been recently described and 
figured by Mr. Etheridge from the Umsinene Eiver deposits. 
This species is represented by nearly a dozen examples in various 
states of preservation, some being partly testaceous and others casts, 
but none showing either muscular scar markings or the characters 
of the hinge. With the exception of a young form with imperfect 
valves which has a height of 15 mm., the specimens may be 
said to vary in that measurement from 23 to 30 mm., whilst the 
type, according to the published figure, exhibits a measurement of 
25 mm. in the same direction. A slight variation is noticeable in 
the general contour of the shell, the antero-ventral end being rather 
broader in some specimens than in others ; in the original the 
anterior end is said to be " comparatively small." The greatest con- 
vexity exists in the antero-median area of the valves, this being 
succeeded posteriorly by a gradual compression which terminates 
in a moderately sized gape with rounded margins. One of the 
specimens (that figured) has the valves partially open anteriorly, 
but in others they are mostly closed in front. 
The anterior end view shows the strongly incurved, approximate 
umbones, in front of which is a prominently wide and excavated 
sub-umbonal area more or less circumscribed by an obtuse ridge 
or carination, which proceeds obliquely from the umbones to the 
antero-ventral corner of the valves, a corresponding obtuse ridge 
being present on the posterior side of the umbones forming the 
boundary to a narrow, lanceolate escutcheon area. 
The ornamentation consists of a series of concentric, equally wide, 
sometimes irregular, depressed ridge-like bands, separated by 
shallow grooves, which are closer together, narrower, and conse- 
quently more numerous at the umbones than elsewhere. 
The shell structure is extremely thin and only occasionally pre- 
served, but one specimen showing parts of an excellent outer surface 
