28 
K. L. Prendergast. 
Diagnosis . — Shell large, subquadrate to oval in outline. Greatest width- 
near anterior margin. Pedicle valve strongly and evenly convex with median 
sinus, with or without cardinal area. Brachial valve slightly convex to con- 
cave with upturned margins. 
Ornamentation essentially spinose. Spine bases elongated posteriorly.. 
Concentric lamellae or wrinkles crossing shell surface. 
Internal features as in Prodactus s.l. Cardinal process vertical to in- 
clined almost to horizontality. Median septum in brachial valve. 
Description . — This species has been described and discussed in Prender- 
gast {Proc. Pog. Soc. Tas.)*, but some special features of the Western 
Australian shells are added here. 
The pedicle valve is strongly convex with steep flanks and indented by a; 
median sinus, which flattens out anteriorly so that the margin may be entire. - 
The area is narrow in some specimens but may become wide. It is divided by 
a definite delthyrium which is partially filled by a triangular projection of 
the brachial valve. The adductor impressions are dendritic and placed high 
in the valve, the longitudinally striated diductors arise at the sides of the 
adductors and are produced laterally and anteriorly. 
The brachial valve is flattish with a slightly convex region near the 
umbo; it is upturned at the margins. The cardinal process is strong and 
trilobcd and varies in its inclination to the plane of the valve from being 
eoplanar to almost perpendicular, the process in this case assuming a horizon- 
tal position. The median sei^tum arises at the base of cardinal process and 
continues forward for about two-thirds of the length of the valve. The adduc- 
tor muscle impressions are dendritic and enclosed in a heart-shaped elevation 
whose apex is at the base of the cardinal process. From the base of this eleva- 
tion arise the brachial impressions, these running parallel to the hinge-line’ 
almost to the lateral bordei’s then turning in a circular path and returning to 
their points of origin parallel to their outward path. At the base of the 
cardinal process arise two laterally directed ridges running parallel to the 
hinge-line but dying out before reaching the ears. 
The ornamentation of the shell varies within rather wide limits. It is 
essentially spinose but the intensity of the spinosity varies from locality to 
locality. In the Irwin shells and those from the North-West Division the 
spines are very fine and closely set on concentric lamellae, in those from the 
Kimberley Division they are much thicker and fewer in number. 
* See footnote p. 27. 
