CORRIGENDA ET ADDENDA. 609 



the shell of the crustaceans ; the resemblance between 

 these and the artificial products is, in some respects, more 

 complete than in that of the calculi. All the appearances 

 in shells can be best observed by merely cleaning them in 

 water, and examining them in glycerine, grinding being 

 unnecessary and injurious. Polarised light is indispensable ; 

 as in the young hermit-crab, at the part where the cal- 

 careous and membranous portions of the shell are con- 

 tinuous, the circular forms of globular carbonate are so 

 delicate that no evidence whatever of its presence can be 

 detected under powerful lenses, and with the best illumi- 

 nation, until polarised light is brought to bear upon the 

 specimen. To obtain the most satisfactory results in the 

 investigation of the process of calcification of animal 

 tissues, it is indispensably necessary that the parts ex- 

 amined should be in the earliest stages of the process, and 

 before the calcifying membrane is entirely covered with 

 the globular coalescing deposit. The usual plan of examin- 

 ing shells in thin vertical sections is entirely useless, 

 unless it be simply to see the number and arrangement of 

 their layers ; the part of the section in such specimens, in 

 which the calcifying process ought to be best seen, being 

 entirely ground off. This part, being the softest, can only 

 be preserved in the process of grinding by extreme care, 

 and by keeping the lower edge of the section always 

 thicker than the upper. 



"The resemblance between dentine and crab-shell, as 

 observed by Dr. Carpenter, is in most respects correct, and 

 sufficiently striking to justify the inference that they are 

 formed in a similar manner. The globular particles of 

 carbonate of lime in shell are doubtless analogous to the 

 globular dentine in teeth, and the apparent spaces between 

 the radiating lines in the former correspond in a great 

 measure to the so-called dentinal tubes in the latter. But 

 respecting the fact of there being in either case tubes, the 

 results of my experience and investigations are opposed to 

 this view. I may observe, that in the end of the crab's 

 claw, as seen by the microscope, there are, as in other 

 parts, two different kinds of structure, one consisting of 

 alternate dark and light, generally sinuous lines, extending 

 from the superficial towards the deep surface of the shell, 



