STONE BORERS. 295 
the siphons when they are extended. Such an 
one is now before me, which has been in my pos- 
GASTROCHZENA. 
session in a living state for several months, during 
which time it has slightly increased the length of 
its projecting tube by the addition of shelly 
matter. Internally the tube gives indications of 
a division into two, but the partition does not 
extend to the middle from either side. The crim- 
son tips of the siphons may often be seen when 
the animal is undisturbed, just reaching to the tip 
of the tube, or projecting in the smallest degree 
beyond it. ‘The investing tube is found, on care- 
fully breaking the stone, to enclose the animal 
with its valves in a sort of flask or bottle. This 
species (G. modiolina) is reckoned among our rarer 
shells. 
GENUS SAXICAVA. 
No accessory tube is formed by the animals of 
this genus, which live in the hardest rocks, espe- 
cially those of calcareous formation. The shell is 
oblong and gaping, with prominent beaks; the 
