M.VLACOSTRACA. 13 



which are terminated by two or three filaments; two mandi- 

 bles, each of which, at its base, bears a palpus that is divided 

 into three joints and usually laid on it; a bilobate tongue; 

 two pairs of jaws ; six foot-jaws, the four posterior of which, 

 in some, are transformed into claws ; and ten feet, or fourteen, 

 in those where the four foot-jaws have that form. 



In the greater number the branchiae, of which there are 

 seven pairs, are concealed under the lateral margin of the 

 shell: the two anterior pairs are situated at the origin of the 

 four last foot-jaws, and the others at that of the feet properly 

 so called. In the other Crustacea they are annexed, in the 

 shape of tufts, to five pairs of paddles (feet) placed under the 

 post-abdomen. The under part of this posterior portion of 

 the body is similarly furnished, in the others, with four or 

 five pairs of bifid appendages. 



ORDER I. 



DECAPODA. 



The head, in the Decapoda, is closely joined to the thorax, 

 and covered with it by a shell, entirely continuous, but that 

 most frequently exhibits deep lines dividing it into various 

 regions which indicate the places occupied by the principal 

 internal organs(l). The mode of their circulation presents 

 characters which distinguish them from the other Crustacea. 



(1) M. Desmarest, in his Histoire Naturelk des Crustace's Fossiles, and in his 

 Considerations Generates sur la Classe des Crustacea, lias presented us, in relation 

 to this point, with an ingenious nomenclature, based on the concordance of the 

 portions of the external surface of the shell with the organs they cover. But, in 

 addition to the fact that the shell of several Decapoda presents no impressions, or 

 has them nearly obliterated, these denominations may be replaced by others 

 more simple, more familiar, and relating 1 to these same organs; as the middle or 

 centre, the anterior and posterior extremities, the sides, &c: it appears useless to 

 increase our nomenclature in this case. 



