GASTROCH ENA. 159 
both surfaces; the pair of palpi, on each side, are subtri- 
angular, poited, smooth without, but striated on the internal 
faces. 
I believe one of the objects of the folds in the ventral 
portion of the mantle is to give greater elasticity and strength 
to that organ, in rubbing out the habitations of these creatures 
when in limestone. Many conjectures have been hazarded, 
how the animals of Gastrochena, as they grow, form and 
increase their flask-shaped habitations of agglutinated particles 
of gravel, sand, and shelly spoil, when they are located in old 
dead bivalve shells. I think the solution of this poimt pre- 
sents no particular difficulty. The enlargement is effected, 
as in the shells of the ordinary bivalves, by the periodical 
additions of testaceous matter, and of internal lining, to the 
gradually progressive excavations or tubular constructions, 
until the animal comes to maturity. It is an error to suppose 
that an animal, at any particular stage of its increment, before 
it has arrived at the full size, becomes completely encased, 
and in consequence must find some mode to extend the 
dimensions of its habitation m proportion to its own in- 
crease ; such an operation is impossible, except by chemical 
or mechanical means; but absorbents and solvents, even if we 
admit their application to calcareous matter, and that they 
have a solvent power not subject to neutralization, certainly 
would have no effect on the tubes that are often formed, of 
flmty and corally spoil. We believe that the fact is, the 
animal in some deposits is never entirely enclosed before it is 
adult, and that it creases its incipient cavity from the point 
on which it is first cast by regular progression until full ma- 
turity, when a final closure takes place, which is the test that 
further enlargement has ceased. We consider these views are 
analogous with the doming of the shell of the Pholas papy- 
racea, which is delayed whilst the animal gradually enlarges 
its habitation until the full growth determines the final 
closure. 
The foregoing remarks have chiefly reference to the pear- 
or flask-shaped excavations and constructions in old bivalves. 
There is greater difficulty to account for the progressively 
