BORERS. 301 
than in the obscure harbour of Portpatrick, which 
has never been visited by a foreign craft?” * 
GENUS PHOLAS. 
The shells of this genus are of delicate and 
fragile texture, and of a pure white hue, when un- 
defiled by extraneous substances. The valves are 
much developed behind, gaping at both ends, but 
especially in front. Their exterior surface is 
rough, with transverse scaly ridges and furrows. 
A spoon-shaped process springs from beneath the 
beaks in each valve, directed forwards. The beaks 
are covered by a callosity, and there are accessory 
plates or valves at the back of the shell. 
The animal is thick and club-shaped, with long 
siphonal tubes united externally into one. The 
foot generally is large, thick, oblique, and flat at 
its extremity. The burrows in this genus are 
never lined with a shelly coat. 
Frequent mention has been made in the pre- 
ceding pages of the siphonal tubes, with which 
many genera of Conchifera are furnished; and it 
has been explained that, whether apparently united 
in one or obviously distinct, these tubes are the 
external organs accessory to respiration. Even 
when apparently united, as in the present genus, 
a glance at the very tip shows that there are two 
openings, one of which is a little smaller than the 
other, and commonly this subordinate orifice di- 
verges at a slight angle from the principai one. 
The latter is the entrance, the former the exit 
for the water, a perpetual change of which is abso- 
lutely indispensable to the life of the animal. The 
* Edin. New Phil. Journ. 1835. 
