66 CRUSTACEA. 



in the efferent channel and branchial peculiarities, as well as in many 

 other particulars, although some species have the buccal area a little 

 narrower anteriorly than behind. 



The number of branchias is less distinctive, as already explained. 



The characters next highest in value, are those of the genital 

 system, especially the position of the male appendages. 



To a certain extent the general form is of great importance, inasmuch 

 as the form is an indication of the position of the internal parts of spe- 

 cies, and preeminently of the greater or less concentration of the organs 

 of the senses. We perceive at once the wide distance between the 

 Maioidea, in which the anterior, across the medial or stomach region 

 is narrow, with the front narrow and prolonged, and the great bulk of 

 the body is posterior to its middle, and the Cancer or Grapsus, which 

 forms have the body as broad before as behind, and no rostral elongation 

 in front. The character of the epistome, and the relation of the outer 

 antennae to the eyes, are dependent on the form. 



Of considerable weight may be the characters afforded by the outer 

 maxillipeds and outer antenna' the articulation of the third and fourth 

 segments of the outer maxillipeds, whether at the inner apex of the 

 former or remote from this apex the size of the outer antennas, 

 whether small, naked, and flexed outward, or large, hairy, or flexed 

 inward. The maxillipeds thus separate the MAIOIDEA and CAN- 

 CROIDEA from the GRAPSOIDEA; and the outer antennas remove the 

 CORYSTOIDEA from the other groups, allying them at the same time to 

 the Hippidea. 



We thus arrive at the grand divisions instituted by Milne Edwards, 

 with the exception of the separation of the Corystoidea from the Leu- 

 cosoidea, of which we propose to make a separate group. In the 

 Maia and Leucosia groups we agree nearly with De Haan, but not in 

 the other groups. 



The five subtribes into which the Brachyura are distributed, are 

 characterized as follows: 



I. CRUSTACEA MAIOIDEA. 



I. Via efferens partes palati laterales trajiciens; area buccalis sub- 

 quadrata. 



II. Pyramis branchialis novem branchiis instructus, septem bran- 

 chiis superficiem pyramidis construentibus. 



