WILLIAMSON, ON SHELLS OF CRUSTACEA. 47 



The laminated structure of these calcareous concretions is 

 beautifully shown by some examples I met with in a very 

 small Australian crab. The fleshy substances in the claws 

 were dried up, but in their interior were numerous botryoidal 

 concretionary masses, the structure of which, as revealed in 

 thin sections, is represented in fig. 19. The concentric lami- 

 nation is here so obvious, that no question can arise as to 

 the mode of increment, whilst the masses appear manifestly 

 to be but enlarged illustrations of what we found on a smaller 

 scale in figs. 7, 8, 9, 10, and 15. Numerous dark points ex- 

 isted in these concretions, as represented in the lower part of 

 the figure, but which are omitted in the upper part, in order 

 to display the completeness of the concentric lamination. 



We are thus compelled, equally from an examination of 

 their mature states, and from their development as traced in 

 the hermit crab, to reject Dr. Carpenter's hypothesis of the 

 cellular structure of the areolated layers of crustacean integu- 

 ments. At the same time, they illustrate how a cellular pro- 

 toplasmic structure may secrete a tubulated tissue, closely 

 resembling dentine, from which it is separated by a struc- 

 tureless basement membrane, through which neither cells 

 nor nuclei ever pass. Whether the cells represented in 

 fig. 18 have anything to do with determining the position of 

 the areolae in the calcified layers I cannot say, though I 

 doubt their doing even that much, since the spaces they 

 occupy do not seem to correspond with those of the areola? ; 

 nevertheless, the question is worth further investigation, could 

 Crustacea, that have just moulted be obtained for examina- 

 tion. 



