CORRIGENDA ET ADDENDA. 611 



ends are shown, these having been most probably ground 

 off in making the preparation, but, in other respects, they 

 perfectly agree with those I am describing, but in no 

 respects with the flexous lines before described. I have 

 thin sections of crab's claw which had not been boiled, 

 showing these tubes, but they are difficult to prepare. 

 They can be most easily shown in the very young crab, 

 but the best way to see them is in the decalcified shell. 

 Now, I can easily imagine that tubes thus constructed, 

 with one of their extremities expanded, and thus rendered 

 favourable for the operation of extensive endosmose or 

 exosmose, and with the other extremity close to the part of 

 the shell where the globular carbonate of lime is chiefly 

 found, would serve admirably to convey the water in 

 contact with the surface of the animal, containing salts of 

 lime, to the membrane lining the shell, where a quantity 

 of sub-carbonate of soda is always present, and thus to 

 bring into operation all the conditions required to form 

 the globular carbonate which occurs in this situation. But, 

 on the contrary, in the ivory of the tooth, circumstanced 

 altogether unlike the claw of the crab, such tubes could 

 be of no use, especially as they would be covered with a 

 layer of enamel, and in some animals with a layer also 

 of cementum. Now, as it is extremely probable that, with 

 organs so decidedly tubular, there would not exist another 

 set of tubes to perform the same function, an additional 

 reason is here afforded in favour of the opinion of 

 the structure of these alternating dark and light lines 

 being as before described. It may be observed, further, 

 that interstices of precisely the same character as those 

 existing between the particles of dentine are found also 

 between the analogous portions of enamel, which are also 

 considered by some anatomists as distinct tubes." 



" Dr. Carpenter says, that he has ascertained that the 

 nacreous lustre of certain shells is due to the plication, or 

 folding of a single layer of this membrane in such a mode, 

 that the folds shall lie over one another in an imbricated 

 manner. Although Dr. Carpenter's evidence upon this 

 point appears very clear and circumstantial, it does not 

 agree at all with the facts which occurred to me in ex- 

 amining the same parts, and therefore I am obliged to 



