INTKODUCTOKY CHAPTEE. 11 



in various ways, but of these the headless 

 Molluscs are destitute. 



Having thus given a short general descrip- 

 tion of the animals inhabiting shells, the next 

 object of inquiry is the formation of shell itself. 

 u The molluscous animals are oviparous, pro- 

 ducing their young from eggs in the same way 

 as birds, although many of the tribes do 

 not, at the time of their exclusion, abandon 

 them at once, but deposit them between the 

 two membranes of the branchial laminae, where 

 they undergo a species of incubation."* " The 

 animal has a small portion of shell attached to 

 it, even before the egg is hatched : while en- 

 closed, the shell is generally of a pale horn 

 colour, and destitute of markings ; when, there- 

 fore, they remain attached to the apex of the 

 spire of adult shells, they may be easily distin- 

 guished by their appearance from the part 

 formed after their exclusion."! As the animal 

 increases in bulk, it finds the shell is not suffi- 

 ciently large to cover it, and it then adds to its 



* From the Penny Cyclopaedia, article Conchifera. 

 f From Mr. Gray, Philosophical Trans. 1833. 



