CONSTRUCTION OF SHELLS. 7 
barren shores of the Frozen sea; and these, though 
extremely elegant in their construction, are plain 
with respect to colour. 
This remarkable difference is owing, undoubtedly, 
to different degrees of solar heat, and probably the 
vast seas of the equatorial regions, which are not 
liable to be weakened by the pouring in of such a 
“liquid weight” of fresh water, as the Oronoque, 
Orellana, and “ sea-like Plata,’ continually bear 
into the Atlantic Ocean, may afford a greater quantity 
of nourishment to such molluscous animals as inhabit 
them, and thus augment the size and brilliancy of 
their testaceous coverings. 
It appears from the accurate investigations of 
Leuwenhoeck and Reaumur, that an infant shell-fish 
is uniformly furnished with a testaceous coating, 
which gradually increases by means of a viscous exu- 
dation from the aperture, or hinge round the circum- 
ference of each valve, and forms a coat of mail, 
shed, or marine pavilion, adapted to the exigencies, 
and proportioned to the shape of the wearer. ‘These, 
as the animals increase in size, increase also in thick- 
ness though not in length ; while the elevations, 
depressions, striz, tubercles, and spines, which 
distinguish individual subjects, may be attributed 
to corresponding projections, tentacula, and other 
irregularities in the fleshy form of the constructing 
