GASTROCH^NA. 159 



both surfaces ; the pair of palpi, on each side, are subtri- 

 angular, pointed, 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 Gastrochana, 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 point 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 in 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 

 flinty 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 increases 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 



