Withers. — NcoacalpcUum in Cretaceous of Western JustraliaAOS 
tother (PI. Xr, figs. 5, 6) in the Geological Department of the British 
tMuseuni, registered In. 25,978. A scutum referred provisionally to 
Ithis species (PI. XI, figs. 1, 2) is in the Western Australian Museum, 
iregistered 4461. 
Measurements . — The holotype is complete and has a length of 
17.8 mm. and a breadth of II mm. Tlie paratype has an incomplete 
length of 19.7 mm. The scutum has a length of 22.5 nun., and a 
breadth of 8,8 mm. 
Description. — Tergum V-shaped. with a prominent, wide, flat 
apico-basal ridge, sloping towards the scutal side, and Avidening 
gradually towards the basal angle, Avhich is obliquely truncated. 
U])per carinal margin short, little more than one-third the length of 
the lOAvcr carinal margin. Scutal margin deeply excavated, leaving 
a narrow limb of the valve on the occludent side, and this is only a 
little wider than the carinal limb. In the larger valve the carinal 
limb is rather narroAver than in the smaller valve, and is only about 
the Avidth of the Avide apico-basal ridge. These terga vary also in 
that the smaller tergum has the carinal limb inclined aAvay from 
the carinal margin, Avhilc in the larger valve it is inclined toAvards 
the carinal margin; this influences the curvature of the apico-basal 
ridge and of the loAver carinal margin. The emargination of the 
scutal margin abruptly takes place in the loAver half of the valve, 
for the earlier stages of groAvth are quite normal. The outer surface 
is marked by obscure longitudinal ridges. 
A scutum from the same horizon as the above terga, but occur- 
ring at One Tree Hill, Gingin, may, in the absence of further evi- 
dence, be referred to this species. It has the following characters: — 
Scutum thick, surface Avith slightly raised groAvth-ridges, crossed 
by obscure longitudinal ridges; triangular, much elongated; strongly 
convex transversely, i-ather more steeply on the occludent side ; umbo 
apical. Occludent margin slightly convex ; tergal margin slightly 
convex; basal margin convex, and extending upAvards in a A\dde 
curve to the tergal margin. On the inner surface the pit for the 
adductor muscle is large and deej^, inid takes ui) almost the Avhole 
of the loAver half of the valve. The inner occludent edge stands 
out prominently and is much pi’oduced just below the apex; oAving 
to the prominence of this ridge at the apex, there is a deep triangular 
depression on the tergal side presumably for the reception of the 
scutal angle of the tergum; the occludent edge is divided by a deep, 
narroAV, longitudinal furroAV for the Avhole length of the valve. 
Eemarhs, and comparison with other species. — Since this type 
of tergum is unknowm among Cretaceous and later fossil species, it 
cannot be compared Avith any, and from the recent species it appears 
to differ in the presence of the Avide, raised, apico-basal ridge. 
