30 THE AUSTRALIAN NATURALIST. 
and the crystals assume the perfect cube and also the pyrito- 
hedron, a modification of the true form. The erystals in the 
specimens from Leadville show remarkable regularity. They 
include both types, and often show the characteristic striae at 
right angles to those of the adjacent face. 
H. R. Drummonp. 
Fulgurites—These objects, also known as “lightning 
tubes,” are comparatively rare. They are occasionally found in 
the sand dunes around the coast, having been formed by 
lightning striking the sand. When lightning strikes the sand 
the resistance offered to its passage generates intense heat 
whereby the moisture in the sand is vapourised, forming an 
open channel or pipe, the sand grains around which are fused 
along the path taken by the lightning. As the lghtning 
travels downwards to the wetter sand, it becomes diffused and 
the walls of the tube become rougher and less perfectly fused. 
Many of the tubes enclose roots of plants which have, grown 
down them subsequent to their formation. In some cases I 
might suggest that the lightning has followed the course of a 
root, as many of the tubes closely resemble roots m_ outline. 
Many of the tubes are decidedly flattened, in fact few are per- 
fectly round in section. This may be accounted for by the pres- 
sure of the surrounding sand when the tube was soft from the 
state of fusion. 
Professor David tells me that he traced a fulgurite down - 
in the sand for fourteen feet and it was still travelling down- 
wards. Those at Port Macquarie, however, do not generally 
extend for more than three feet below the surface. The fused 
siliea appearing as a glassy substance with bubbles visible in it 
may be seen on the inside of most of the tubes. 
HeatHer R. Dru Monp. 
PINNOTHERES PISUM, A PARASITIC CRAB. This elegant little 
crab is parasitic between the valves of the common Pipi (Donax 
deltoides). An interesting feature is that the male is unknown, 
those found always being females and frequently carrying eggs. 
The abdomen is relatively unusually large, more so than in most 
other erabs. The actual location in the pipi is in the mantle 
cavity. The specimens shown were from Port Stephens. 
P. D. Murray. 
Arrencuyma. The aeration of plants is made possible by 
means of intercellular spaces between the cells. The stomates of 
