44 OSTREIDiE. 



separated by a wide chasm. The shell is remarkably 

 calcareous, and consists for the most part of layers 

 termed by Dr. Carpenter " sub-nacreous " and having 

 comparatively little adhesion one to another. These layers 

 are internal. The outer layers are composed of pris- 

 matic cellular structure, and have no natural cohesion. 

 The weight of the animal in a full-grown example is 

 very disproportionate to that of the shell. The late Mr. 

 Thompson of Belfast ascertained that a large oyster 

 from that bay weighed altogether two pounds, but that 

 the weight of the animal taken out of the shell was only 

 an ounce and a half. Large-sized specimens from the 

 British seas seldom exceed six inches in length ; but on 

 the North-American coast this species (if it be the same 

 as ours) is said to attain occasionally twice that size. 

 Young shells are sometimes marked with radiating 

 purple streaks ; and now and then one is found attached 

 to the operculum of a living Buccinum undatum, the sur- 

 face of which it completely covers and takes its form. 



Before adverting to the economical point of view, I 

 may mention some of the minor uses to which oysters 

 are put. These are few : they serve to keep an aqua- 

 rium free from the spores of sea-weeds ; their shells are 

 burnt as a substitute for lime ; and formerly certain 

 medicines were prepared from their calcined material. 

 Also pearls of inferior lustre, often small and of an 

 irregular shape, are obtained from them. Antiquaries 

 tell us that the shells have been discovered in Saxon 

 tombs, and that in still older places of sepulture in the 

 Orkneys they are found drilled in such a manner as to 

 show that they probably formed articles of personal 

 ornament. They must have made a clumsy necklace. 

 But their »chief value results from the fisheries, which 

 for more than eighteen centuries have rendered Great 



