28 On the Form, Growth, and Construction of Shells. 



abound in the fine sediment brought up by the dredge from 

 great depths. A few species occur in the Tertiary strata of 

 England and the continent; in the older rocks they are 

 unknown, unless some comparatively gigantic forms (Gonularia 

 and Theca) have been rightly referred to this order." (Wood- 

 ward's Manual of Mollusca, p. 202.) 



It has been stated that the specific gravity of floating shells 

 is lower than that of any others, and such may be the case with 

 "the Paper Nautilus/' Argonauta, Ianthina, Garinaria (see 

 Plate, p. 21, Fig. 3), and Pteropods (see Coloured Plate, Vol. 

 x., p. 241, Fig. 1). But none of these will float in the water 

 by themselves. Ianthina is rendered buoyant by the air- 

 vesicles attached to its foot, and the others are sustained by 

 incessant muscular exertion in swimmiug. The Nautilus and 

 Spirula are in no degree indebted to their specific gravity, but 

 solely to the air-chambers with which their shells are furnished. 

 For the shell of the Spirilla is wholly composed of nacre, and 

 the pearly lining of the Nautilus constitutes the greater part 

 of the thickness of the shell. Nacre, we have seen, is Arago- 

 nite in its physical character, and has a higher specific gravity 

 than the ordinary shell. The internal pen of the Calamary 

 (Loligo vulgaris) is composed entirely of Conchioline, and is 

 consequently lighter than any floating shell ; but it would sink, 

 nevertheless, if deprived of the air it contains. The " cuttle- 

 bone," or shell of the Sepia, swims, because it is full of air. 

 The author has seen it floating in the middle of the Bay of 

 Biscay, and hence, no doubt, that many -such shells are wafted 

 by " RemelPs current" from the coast of Portugal, and stranded 

 on our own south-west shores. Professor Forbes remarked 

 that he had never dredged a cuttle-bone, but multitudes were 

 found cast ashore on all the coasts of Asia Minor. 



The Spirula has never been taken alive, and rarely with 

 any portion of the animal attached to the shell ; but it must be 

 abundant in the tropical Atlantic, as well as in the Coral Sea. 

 In Brazil and the West Indies most likely, is its home. 

 Millions of the shells, floated across the Atlantic by the Gulf 

 Stream, are strewn on the shores of the Canary Islands. 

 They are less common at Madeira and along the Iberian coast, 

 and very rare in Devon and Ireland. 



The dead shells of Nautilus abound in the Coral Sea, and 

 are cast ashore in such profusion that many tons weight are 

 collected at New Caledonia and the Feejee Islands, and con- 

 veyed to Sydney, where they sell for three halfpence each ; or 

 to the Navigator and Friendly Islands, where, not living, they 

 are worth one shilling a piece. The young shells, of which 

 ornaments are made, when polished, fetch a high price. 



It seems superfluous to endeavour to explain the " floating " 



