INTEODrCTIOX. 



called nacre, or motJier-of-pearl, is mostly fomicl. One of the 

 commonest examples is the oyster shell; if broken across it will 

 be seen to consist of very thin plates, or lamina, as they are 

 termed, closely packed together. The thinner tliese laminae may 

 be the more lustrous and beautiful appears the lining of the shell; 

 that shifting play of colours which we call iridescent, from iris — 

 the rainbow, is then brightest and most noticeable. A veiy re- 

 markable substance is this mother-of-pearl; smooth, and shining, 

 and delicately-tinted. Who would expect to find such a beautiful 

 lining to the rugged, rough, dingy-looking oyster, or mussel shell? 

 Truly these mollusks, some of them, live in gorgeous palaces. And 

 the most curious part of the matter is that from the fluids or juices 

 of their own bodies, and from the chalky matter collected from 

 the water, they are enabled to secrete or deposit such wonderfullj'- 

 constructed habitations, which after all are little more than chalk. 

 Burn a heap of oyster shells, or any other testaceous coverings, 

 and you get lime the same as that produced by burning the 

 white lumps from the chalk-pit, which lumps, by the way, are 

 said to be composed wholly, or for the most part, of marine shells. 

 This we should call cretaceous matter, from creta, which is the 

 Latin for chalk, or calcerous, from calcis — Ume. Granular shells 

 you have been told are sometimes called concretionary, this is 

 because they contain a large amount of this chalky deposit. 

 The rock called limestone, geologists tell us is composed entirely 

 of fossil shells and mud, or what was once mud, dried and hard- 

 ened, most likely by extreme heat, to the consistence of rock. 

 Wonderful this to think of; huge mountains, and mighty masses, 

 and far-stretching strata, forming a large portion of the crust of 

 the earth, made up chiefly of the coverings of fishes, a great 

 portion of them so small as to be scarcely visible to the naked 

 eye. — Truly wonderful! But we shall have more to say upon this 

 head when we come to speak of Fossil Shells, as well as on the 

 subject of Pearls, in our chapter on the fish in whose shells they 

 are chiefly found. 



It has been a matter of dispute with naturalists whether the 

 testaceous moUusks have shells at all before they issue from the 

 egg, and the main evidence favours the opinion that, generally 

 speaking, they do possess what may be considered as a kind of 

 pattern or model of the habitation which they are to build. This 

 appears to be of a pale horn-colour, and destitute of any markings ; 

 but as soon as the animal enters upon an independent state of 

 existence, it begins to assume its distinctive shape and colour, 

 gradually increasing with the growth of its living tenant, and 

 becoming more and more decidedly marked, until it attains its 

 full perfection of testacean development. Thus the age of some 



