MICROSCOPIC OBJECTS. 61 



is deposited. Some naturalists have considered shell 

 analogous to the bone of animals, and hke it receiving 

 nourishment from the animal ; while others liken it to the 

 horny structures of animals, as the hoof, nails and hair, 

 which when fully developed cease to hold any vital com- 

 munication with the animal. 



Shell has been classified variously by different observers. 

 Mr. Hatchett separated it into two groups, which he 

 called Porcellanous and Nacreous, the carbonate of Ume 

 in the former being accompanied with very little animal 

 matter, while in the latter the animal matter is so abundant 

 that after the earthy matter is dissolved by dilute hydro- 

 chloric acid there remains a perfectly definite membranous 

 animal residuum. Another division of shells has been 

 founded on the manner in which the carbonate of lime 

 has been deposited by the animal, some shells having a 

 crystalline fracture, others a granular or concretionary 

 fracture. According to the experiments of Dr. Carpenter, 

 undertaken at the request and expense of the British 

 Association, the calcareous matter in all sheUs is nearly 

 equally crystalline in its aggregation, and the particular 

 forms which their fracture presents are determined chiefly, 

 though not entirely, by the arrangement of the animal 

 basis of the shell, which possesses a more or less highly 

 organized structure. 



All thin sections of recent shell are translucent, except 

 when the colouring-matter is opaque, or when the calca- 

 reous matter is deposited in a chalky state between the 

 true laminae of the shell, as in the oyster. All thin sec- 



