a Shells 
Je New Zealand 
Plate X 
No. 10 
the hinge towards one end. For the size of the shell, it is 
highly convex and has strongly-marked radiate ribs at 
either end, extending from the hinge and spreading out 
towards the ventral border, leaving a comparatively smooth 
space in the middle. It is a beautifully coloured shell, 
being of a pale yellowish green at the hinge, while the 
ribbed portions and the smoother central portion are of 
a rich olive green. The interior is iridescent, with a lovely 
play of colours, and an underlying tinge of brown. It is 
about an inch and a half in length. The M. impacta is 
found from low water to a depth of twenty-five fathoms, 
and live specimens may be obtained by carefully searching 
among boulders at low spring tides. A cluster of perhaps 
a dozen of all sizes may be seen hidden in a dense mass of 
tangled and matted beards or byssus threads. So closely 
are the mollusca packed, and so dense the dark-brown — 
beards which bind the colony together and secure it to the 
rock, that it might easily be mistaken for some vegetable 
growth, and thus escape the notice of the inexperienced 
collector. When dried, the mass of byssus threads has the 
appearance of soft and fine brown wool. 
North and South Islands. Mount Maunganui; Chat- 
ham Islands. 
LITHOPHAGA TRUNCATA (lithos, stone; phago, 
to eat; truncata, cut off, or lopped at the top).—Com- 
monly known as the Date shell, this species is a rock- 
borer; a thin, polished, elongated bivalve, more than twice 
as long as it is broad, and exactly the same colour as the 
date fruit. The sculpture consists of concentric striations, 
with a few indistinct radiate lines from the ventral border 
towards the hinge, which, however, they fail to reach. 
The fresh shell is slightly iridescent internally and bluish 
white, purplish at the posterior end. This borer is attached 
to the rock by a beard when young, like the mussels. The 
tunnel formed by the animal is of the same shape as the 
shell, there being no rotary movement, and consequently 
the vaives are practically smooth. Most rock-borers rotate 
their shells, and in order to perform this manceuvre are 
provided with roughened and rasp-like valves. Of course, 
the result is the same, but in the case of the Date shells 
the tunnels are excavated by the foot, whereas the other 
rock-borers use their valves as cutting and boring imple- 
ments, the foot supplying the motive power. When in 
soft sedimentary rock and above low-water mark, they 
may be obtained by chopping out blocks with a tomahawk, 
and fine specimens in situ can thus be secured. The rock 
should be trimmed to a suitable size and shape, and the 
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