218 THALASSINAD.E. 



racterized by the semi-membranaceous texture of the 

 external skeleton, by the remarkable length of the ab- 

 domen, the compressed form of the carapace, the absence 

 of any laminar appendage to the external antennae, and 

 other striking characters. The family is divided into 

 two distinct groups, according to the structure and situa- 

 tion of the respiratory organs. All the British species 

 belong to that division in which the branchiae are wholly 

 contained within the usual branchial cavity under the 

 margin and sides of the carapace, and which are with- 

 out any branchial appendages to the under surface of the 

 abdomen. 



Of the genus Callianassa there is but one British 

 species. 



