Boring Mollusca of St. Andreics. 371 



cannot bore, or even enlarn;e an existing burrow to any 

 Appreciable extent, though they may in some degree deface 

 its walls by friction. The creature is very active in its 

 movements, and is probably able to move to a new habitation 

 when it needs a larger one. Its presence in a burrow is 

 comparatively exceptional ; many specimens are found free. 

 I suspect that the distorted shells belong to specimens which 

 have passed the winter in shelter, and having grown a little 

 during that period, have not been able to get out again. 

 The shell gives the calcite reaction with Meigen's wet 

 test. 



Saxicava rtigosa also utilizes old burrows of Zirphcea, but 

 it is quite able to make a burrow for itself, and is often 

 found burrowing in sandstone. If disturbed in its hole its 

 shell may be heard grinding as it draws itself further in. 

 It attaches itself by suction, when frightened, and cannot be 

 removed with the finger, even when a byssus is not present. 

 Its boring movements do not seem to be definite in direction 

 and purpose, like those of Zirphcea. Many of the burrows 

 are parallel to the surface of the stone, so that the animal 

 lies on its side ; this fact indicates that the creature is com- 

 paratively little specialized as a borer. 



The Zirpha'a may perhaps claim to be the most highly 

 specialized of the Plioladide borers. Its method of working 

 is double: it sucks and scrapes; it might be described as a 

 combination of a nutmeg-grater and a vacuum-cleaner. The 

 following are the successive steps by which the animal 

 prepares itself to bore : — 



(1) The foot is extruded. 



(2) A wide gap appears between the foot and the mantle. 

 This gap is not, to any great extent, due to the fact that the 

 width of the foot is somewhat decreased as it is pushed out. 

 For a long time I could not make out the meaning of this 

 movement, which precedes the extrusion of the mantle. It 

 is that the animal is inilating itself, and becomes wider, so 

 that the mantle and shell-valves are carried outward away 

 from the foot. This is the aspect of things as seen in looking 

 at the pedal end, while at the margin of the shell it is seen 

 that the valves are beginning to gape a little more. 



(3) The mantle becomes fully extruded. Extended far up 

 on the upper border of the shell it forms, with the border of 

 the dorsal shield and the accessory pieces of the shell, a 

 dorsal plugging of the hole, while on the lower side, just 

 opposite, it IS firmly pushed out to form a ventral plug. The 

 site of the latter is indicated by that curious narrow border 

 between the pedal area and the siphonal area of the shell, 



