230 
Pholas a transverse shell) and the want of the shelly tube, 
to distinguish them. The form of the animal is very dif- 
ferent, that of the Teredo extending far beyond its proper 
shell, and having at its extremity two spatulate valves; 
whereas the animal of the Pholas is confined to its shell, and 
has not these additional valves. They both bore into 
wood, and apparently by a similar process; that is, by 
repeated semi-circular turns backwards and forwards while 
the valves are partly opened, cutting the wood with the 
striated or rough posterior part of their margins, which as 
they wear blunt, are succeeded by fresh additions at their 
edges; the cutting part may be compared to a series of files, 
bent so as exactly to fit the size of the holes they are re- 
quired to make; now, as the animal grows older, he re- 
quires a larger hole to live in, and the files being renewed 
along the edge of the shell, are increased exactly in the 
same proportion as he grows, and the degree of their aspe- 
rity is proportioned to the substance they are to bore, so 
that each one may cut a sufficient space for itself to turn 
round in before it is worn away: consequently, in the 
Teredo, which only bores wood, the striz on the posterior 
part of the shell are very fine, and cut into minute teeth ; 
while in those species of Pholas which bore into stone, there 
are rows of prominent angular reflected teeth, presenting 
strong sharp points to the surface they are to act upon. 
The motion of the Teredo is probably confined to a smaller 
part of a circle than that of the Pholas, in consequence of 
the shelly part of its tube being fixed. 
These observations are made upon examining the only 
two species of the Genus Teredo with which I am ac- 
quainted, 'T. navalis, Linn. and the fossil one. Among 
the specimens of the first that have fallen under my inspec- 
tion, I have met with none which show any of the accessory 
valves, except the tube and the two spatulate valves attached 
to the animal, nor even the membrane that is preserved in 
