225 
as the same species, as is also 7’. acuticostata, McCoy (Memoirs 
Austr. Mus., iv., part 5, 1902, p. 301). 
The late Mr. Dennant kindly compared our South Aus- 
tralian shells with McCoy’s fossil form from Victoria, and 
he thought they were not identical, nor conspecific with 7. 
reticulata, Ten. Woods. Wowever, I think it is too closely 
allied to be regarded as a distinct species, so it has been 
placed as a variety of 7’. margaritacea, Lamarck. 
Among the many hundreds of valves and living speci- 
mens dredged by me, only 1 has shown any marked departure 
from the type of this variety. This was taken alive at 23 
fathoms in St. Vincent Gulf, together with 2 others, which 
quite conformed to the type. This unique individual exactly 
resembled a half-grown specimen of the common 7. margari- 
tacea, Lam., obtained from Tasmania. The only other aber- 
rant from our form seen by me is a specimen taken by Mr. 
Zietz, at Corny Point, Spencer Gulf. It has the spines of 
7. margaritacea and the same number of ribs, but the shape 
is more that of 7’. Jamarckii, found in Port Jackson, only with 
fewer ribs and fewer spines, and with an even longer and 
more concave posterior border, in which last character it 
departs greatly from our South Australian variety. The oc- 
currence of these two marked aberrants among numerous 
examples of the ordinary form indicates some other explana- 
tion for oyr varjety than either locality or depth of water 
[Dre ppener7 ed ag n. sp. Pl. xxviii, figs. 15, 16, 17,18. A 7g, 
Shell small, thin, ovate, ventricose. Beaks apposed, in- 
flated, oblique. Post-dorsal line nearly straight, 11 mm.; 
anterior-dorsal line, 2 mm. Ventral border ‘anteriorly 
slightly convex for 8 mm.; then rather more convex, and 
merging into the semicircular posterior end, which, with a 
| more open dorsal sweep, forms a faint rounded angle with 
the post-dorsal line. A heart-shaped area, 10 mm. by 8 
mm., on the united valves anteriorly is bare and glistening 
and sculptured with very low lamellar accremental strix. 
The rest of the shell is very closely invested with a felted 
periostracum of short hairs, with short, blunt projections on 
each side. Internal border simple. Horn-coloured, lighter on 
the umbonal ridge, Internally chestnut-brown in front, and 
with pinkish-brown concentric bands, most marked, in the 
dorsal two-thirds, elsewhere a bluish-white. 
Length, 21 mm.; breadth, 13 mm.; thickness, 11 mm. 
It differs from M. australis, Lamarck, in its shorter, 
rounder, anterior end, its greater proportional breadth, and 
its rounder posterior end. Its hairs are more numerous and 
close-set, shorter, and “elk-horn” in shape, instead of simple. 

