ORDER DECAPODA. 163 



The f]fth section, that of Trigona, is composed of species 

 whose testa is generally triangular or subovokl, narrowed into 

 a point, or in the manner of a beak in front, usually very un- 

 equal and rough, and the eyes are lateral. 



The epistoma,or interval comprised between the antennoe and 

 the buccal cavity, is almost always square, as long, or almost 

 as long, as broad. The claws, or at least those of the males, 

 are always broad and elongated. The following feet are very 

 long in a great number, and sometimes even the last two 

 have a different form from the preceding. The third articula- 

 tion of the external jaw- feet is always square or hexagonal, in 

 those at least, whose feet are of the usual length. 



The apparent number of the segments of the tail varies. 

 In many of them it is seven in both sexes ; but in others, or 

 at least in the males, it is less. 



Many of these Crustacea have been vulgarly designated 

 under the collective name of Sea-spiders. 



Although the species of this tribe are very numerous, but 

 two have as yet been discovered in the fossil state, and one 

 of which (Maia squiiiadoj is still existing in the same 

 localities, 



A first division will comprehend those, whose second and 

 following feet are similar, and the size of which diminishes 

 progressively. 



Among those we shall form a first group, of all the species 

 whose tail, either in both sexes, or in the females, consists of 

 seven articulations. The third articulation of the external 

 jaw-feet is always square, and truncated or emarginated at 

 the upper internal angle. 



Very large claws, especially when compared with the other 

 feet, which are very short, directed horizontally, and per- 

 pendicularly to the axis of the body, as far as the carpus or 

 articulation preceding the hand, afterwards folded in front 



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