1889.] REY. O. P. CAMBRIDGE ON NEW ARANEIDEA. 39 
lichen (?) growth that is common on dry soils at this time of year. 
As seen from above, the door is a square, with its two anterior angles 
rounded off, the straight side or base forming the hinge-end. But 
if the door is opened and viewed from below, it has a circular 
appearance, reminding one of a gun-wad, which is due to its white 
lining having been worked into a circular form to fit the mouth of 
the cylindrical burrow. In cross section the door is conical, like a 
plug, with its lower surface convex, like the bottom of a decanter- 
stopper. This surface is beautifully worked over with a network 
of fine, tough fibres, into which the spider (as I have seen) inserts 
its claws to keep the door closed against the entrance of an enemy. 
In no other part of the burrow-lining is this network of fibres to be 
seen. The door is always made thin at the hinge-end, and thick at 
the forward end, the average respective thicknesses being one 
sixteenth of an inch and one quarter of an inch; so that its section 
is wedge-like. The result of this construction is that the door will 
always on release after being opened fall down by its own weight, 
fitting with exceeding and surprising accuracy into the mouth of the 
burrow. So preponderating is the weight of the door at its forward 
end, and so instantaneously does the action of gravity cause it to fall 
when released after being held wp, that the shutting of the door 
closely simulates the action of a spring, and it is very difficult at 
first sight to realize that no elastic force exists in the action. Thus, 
when a spider sitting at the mouth of its hole, with the door ajar, 
vesting on its back, darts down its burrow when startled, the door 
seems to snap down with the action of a suddenly released spring ; 
but that this is really caused by gravity anyone may convince 
himself by cutting the mouth of a burrow out of the ground, and 
noticing the action of the door and its hinge when held upside down. 
‘The hinge, which is beautifully flexible, is formed by a prolonga- 
tion and local thickening of the lining of the burrow, which is also 
carried over the lower surface and round the edges of the door. 
The part of the lining forming the hinge is thick and tough, and of 
the same colour outside as the ground, but there is nothing special 
or mechanical in its structure asa hinge. It may be mentioned here 
that the lining of the tube is thinned off at the mouth of the 
burrow to receive the door, a distinct rim being usually observable 
at the commencement of the burrow proper. There are generally a 
few withered grass-blades worked into the edge of the door, or into 
the edge of the mcuth of the burrow, so as to form a kind of semi- 
circular fringe, which often catches a practiced eye and leads to the 
detection of the hole. The grass-blades are probably inserted to aid 
in assimilating the outside of the burrow to its surroundings, a 
purpose in which they certainly fail, so far as the human animal is 
concerned. In a few cases I have also noticed grass-blades worked 
into the general surface of the door, and at this season, when the 
grass is everywhere withered, these certainly aid in its concealment ; 
but during the rains, when the adjacent grass is green, one would 
think that yellow withered grass-blades on or near the burrow- 
mouth would tend to make it conspicuous. 
