hinge, and which is known as the resilifer, This pit is 
also well developed in the M. subtriangulatum, the Mactra 
discors, and the Cochlodesma Angasi. In the recent state 
it is filled with a dark brown cartilaginous material, which 
is termed the resiliwm. When these shellfish are boiled, 
the valves spring apart, due to the adductor muscles (which 
adduct or draw together the valves during life) losing 
their contractile power, and the resilium, like a piece of 
india rubber, being no longer under pressure, expands to 
its normal size, and so causes the valves to be pushed 
apart. In those bivalves not possessing a resilium, the 
ligament acts as a “C” spring, always in tension, the ad- 
ductor muscles drawing and holding together the shells; so 
we find that, although the opening of a bivalve during life 
is primarily controlled by the will of the animal, the move- 
ment consists of two distinct, yet perfectly blended acts— 
namely, voluntary relaxation of the adductor muscles, 
accompanied by involuntary and automatic expansion of 
the hinge cartilage. 
The Toheroa is a favourite dish with some people, and 
makes a remarkably good soup. It is found buried in the 
sand between tide marks. 
North and South Islands. Muriwai, on the west coast. 
Mount Maunganui. 
MACTRA DISCORS (macira, a kneading trough; dis- 
cors, different).—This species, known as a Trough shell, 
is a large round, rather smooth, white or greyish-white bi- 
valve, almost symmetrical, with a light brown epidermis, 
more or less persistent, near to the ventral margin. The 
sculpture consists of minute concentric striations, absent 
on the beaks, which are white and smooth, gradually be- 
coming more marked as they approach the ventral border, 
where they appear as step-like ridges. The interior re- 
sembles beautifully glazed white porcelain. It is about 
three and a-half inches across. Living specimens are 
washed ashore on sandy beaches after heavy gales, when 
they are eagerly gathered by the Maoris. 
North and South Islands from the Bay of Islands to 
Preservation Inlet. Mount Maunganui. 
MACTRA ELONGATA (mactra, a kneading trough; 
elongated) —An elongated oval, thickly-built, yellowish- 
brown bivalve, speckled with darker brown spots round the 
ventral portion of the shell, but white where denuded of 
the epidermis. The anterior end is small and rounded, 
and the posterior end largely rounded; sculptured with fine 
concentric ribs towards the ventral margin; smooth near 
101 
Plate XII 
No. 5 
Plate XII 
No. 6 
Sea Shells 
of New Zealand 
