Sea Shells 
of New Zealand 
Plate XII 
No.7 
Plate IX 
No. 16 
Plate XII 
No.8 
the beaks. The margins are smooth and fairly thick, and 
the shell has both valves in contact all round when closed. 
The interior is yellowish white, polished and dense, with a 
large triangular pit at the hinge for the reception of the 
resilium. This Trough shell is large, and may attain a 
length of four and a-half inches by two and three-quarter 
inches in height. The shell is more compressed than the 
M. discors—that is to say, it is flatter from valve to valve. 
Found in the North and South Islands, but “nowhere 
common. Washed up occasionally after gales.” (Suter). 
Mount Maunganui. 
MACTRA OVATA (macira, a kneading trough; ovata, 
oval).—Another Trough shell, large, rotund, thin, asym- 
metrical, and yellowish white in colour. It is bluntly 
rounded at the anterior end, and more sharply rounded at 
the posterior end. Sculpture consists of small concentric 
striations of varying sizes. The valves gape at the posterior 
end. The interior is white and slightly polished; the mar- 
gins are thin and sharp. It inhabits muddy estuaries, man- 
grove saltwater swamps, and is three inches in length and 
a little over two inches in height. This shell must not be 
confused with the M. discors, which it somewhat resem- 
bles. The latter is thicker and heavier, has a smooth in- 
terior, and the valves fit closely all round. 
North and South Islands. Auckland Harbour; Pilot 
Bay, Mount Maunganui. 
MACTRA SCALPELLUM (macira, a kneading trough; 
scalpellum, dim of scalprum, a knife)—This mollusc is 
also a Trough shell, but so different to the other species 
already described that one would scarcely believe it to be 
a Mactra. It is a small, fan-shaped, thin, flattened bivalve, 
smooth, white and polished. It is readily recognised on 
account of its shape, being most symmetrical, with the 
beaks in the middle of the dorsal margin, which slopes 
away equally on each side, enclosing an angle of 130 de- 
grees. The ventral margin forms a broad, circular sweep. 
The sculpture consists of very fine and lightly-marked con- 
centric striations. It is barely an inch in length, and is 
found in sand and disintegrated pumice washed up on 
ocean beaches. 
Great and Little Barrier Islands; Hauraki Gulf; 
Mount Maunganui; Cuvier Island. 
SPISULA AEQUILATERALIS (spisula, dim. of 
spissa, thick; aequilaterialis, having equal sides).—A rather 
large, solidly-built, equal-sided triangular bivalve of a white 
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