18 
K. L. Prendergast. 
The shells are small to moderate in size as may be seen from the table 
of dimensions. They have an irregularly hexagonal outline when viewed 
from the pedicle side, with the hinge-line the greatest width of the shell. 
The pedicle valve has a fiattish visceral disc which, following the geni- 
culation of the shell, passes into a regularly curved trail. Transversely the 
shell arch is high, indented medially by a strong median sinus and falling 
gradually to the lateral margins in a slightly convex slope. The umbo is 
small, pointed and slightly overhanging the hinge-line. The ears are small 
and pointed, they have a convex profile in longitudinal section. The orna- 
mentation is reticulate on the visceral disc, the reticulation sometimes irregular 
due to the enlarged rugae on some specimens. The rugae do not develop 
anterior to the geniculation, the trail carrying costae and scattered spine 
bases. The latter are rarely numerous, not more than 4 or 5 being present. 
The co.stae converge slightly towards the sinus giving a very distinct and 
characteristic appearance to the shell. 
The brachial valve is trapezoidal in outline, the long straight hinge-line 
and the anterior margin being the parallel sides. It is regularly concave 
with a median fold which expands towards the anterior margin. The ears 
are excavated to fit tightly with those of the pedicle valve. The ornamenta- 
tion is reticulate over the jDOSterior third of the shell, the rugae then develop 
irregularly and the ornamentation of the rest of the shell may be described 
as costate. The costae radiate from the umbo, those in the central part of 
the shell converging towards and finally coalescing on the median fold. 
The internal features of this s]iecies are not shown by any of the 
Western Australian specimens. 
Comparison with other species . — This species is distinct from other 
Western Australian scmireticulate productids in its small size. The converg- 
ence of the costae towards the sinus in the pedicle valve and the fold in the 
brachial valve serve to distinguish it from other small members of the semi- 
reticulate group. 
In the Western Australian specimens the convergence of the costae is 
not so marked as in some of the Indian and Timor forms. Examination of 
the Timor specimens shows, however, that there is a large range of variation 
in the extent of the convergence. 
Remarks . — As the internal features of this species are not known the 
generic placement is insecure. In external appearance the shells are similar 
to Marginifera Waagen, and knowledge of the internal structures is neces- 
sary before it can be said that they do not belong to that genus. 
Dictyoclostus spiralis (Waagen). 
PI. ii, figs 8, 9. 
1S84-. — Pivdvctus spiralis Waagen, p. 681, pi. Ixvii, fig, 6; pi. Ixviii, fig. 3; 
pi. Ixix, figs. 1 1 2, 3. 
1916. — Prodnetus spiralis Waagen, Rroili, p. 11, pi. cxvii, figs. 1-5. 
1927. — Pradiictus spiralis Waagen, Piener, p. 26. 
1928. — Productus spiralis Waagen, Hamlet, p. 16. 
Material. — Axis. Miis. F 36513, 36514, 36515, W'yndliam Gap, North-West 
Division, Western Australia. Callytharra stage. 
Diagnosis . — Shell large, regularly curved through 270°. Hinge-line equal 
to greatest width. Ears large and reflexed. Pedicle valve highly inflated with 
arched umbonal region, sides parallel and .steep. Ornamentation reticulate on 
