DECAPODA. 19 



They usually inhabit water, but do not instantly perish when 

 deprived of it; some species even pass a part of their lives on 

 land, only visiting the water in the nuptial season, and for the 

 purpose of depositing their spawn. Even they are compelled 

 to fix their domicil either in burrows, or in cool, damp places. 

 The Decapoda are voracious and carnivorous. Certain spe- 

 cies even penetrate into cemeteries, and devour the dead. 

 Their limbs are regenerated with surprising promptitude, but 

 it is requisite that the fracture be at the junction of the arti^ 

 dilations, and when accident determines it otherwise, they 

 know how to apply a remedy. When they wish to change 

 their skin, they seek a retired and solitary spot, in order to 

 be sheltered from their enemies, and to remain at rest. When 

 the change is effected, their body is soft, and has a more ex- 

 quisite flavour. A chemical analysis of the old shell proves it 

 to be formed of the carbonate and phosphate of lime, united 

 in different proportions with gelatine. On these proportions 

 depends the solidity of the shell : it is much less thick and flexi- 

 ble in the latter genera of this order, and further on, it becomes 

 almost membranous. M. de Blainville has observed that the 

 shell of the Palinurus is composed of four superincumbent lay- 

 ers, the superior and two inferior of which are membranous ; 

 the calcareous matter is interposed between them, forming 

 the fourth. Exposed to heat, the epidermis becomes of a more 

 or less vivid red, the colouring principle being decomposed 

 by boiling water; other combinations of this principle produce, 

 in some species, a very agreeable mixture of colours, that fre- 

 quently border on blue or green. 



The greater number of fossil Crustacea hitherto discovered 

 belongs to the order of the Decapoda. Among those of Eu- 

 rope, the oldest approach to species now living in the vicinity 

 of the tropics; the others, or more modern ones, are closely 

 allied with the living species of Europe. The fossil Crustacea 

 of the tropical regions, however, appear to me to bear the 

 closest similitude to several of those now found there in a liv- 

 ing state, a fact of much interest to the geologist, should the 

 study of the fossil shells of those countries, collected from the 

 deepest strata, furnish a similar result. 



