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and boring marine animals. 
into which they formally burrowed, were gradually broken 
up by the sea, until they became so thin that the Pholades, 
before they were full grown, were stopped by the gravel 
and perished. 
Having been unable to procure the living animal, I can 
offer to the Society only the results of my observations on 
some specimens sent me from the Mediterranean. 
Every peculiarity of structure upon which the boring 
power of the Pholas has been shown to depend, exists in an 
equally marked degree in the Teredo. We find the thin and 
obliquely placed ligament of Pholas Candida, the superficial 
muscle connecting the valves before the hinge, the posterior 
adductor and the muscles passing from the lateral processes 
to be inserted into the foot. The muscles indeed vary in their 
relative size, because their size is proportioned to the force 
they are required to exert, which differs in the two animals. 
Most species of the Pholas have comparatively large shells, 
and burrow into substances of but a moderate degree of hard- 
ness ; while the shell of the Teredo is very small, and it 
penetrates the hardest wood. The office of the anterior ad- 
ductor being only to close the valves, its size is in propor- 
tion to the weight of the shell. It is by the contraction of 
the posterior adductor that the animal effects the stroke ; and 
the force of this muscle will therefore depend upon the 
resistance to be overcome. Hence, in the Teredo, the an- 
terior adductor is much smaller than in the Pholas, while 
the posterior adductor is far more powerful in proportion. 
The lateral muscles are more distinct than in Pholas Candida, 
but they are very short. It is not probable therefore that 
