176 CLASS CRUSTACEA. 



ascend upon the back, but the last two alone are upper. The 

 forceps are compressed, almost in the form of an inverted 

 triangle, denticulated, and with the fingers abruptly bent. 

 These Crustacea have the greatest relations with the albu- 

 mea of Fabricius, the first subgenus of the next family, and 

 thus make the passage fi'om the brachyura to the macroura. 

 From the approximation of the feet it is even probable that 

 the genital apertures of the female are situated as in ma- 

 croura. 



The second family, or 



The Macrourous Decapods — Exochnata, Fah., 



Have at the end of the tail some appendages, forming most 

 frequently on each side a fin, and the tail as long at least as 

 the body, extended and discovered, and simply curved to- 

 wards its posterior extremity. Its under part most frequently 

 presents in both sexes five pair of false feet, each terminated 

 by two laminae, or two threads. This tail is always composed 

 of seven distinct segments. The genital apertures of the 

 females are situated on the first articulation of the feet of the 

 third pair. The gills are formed of vesicular, barbed, and 

 hairy pyramids, and disposed, in many, either on two ranges 

 or by bundles. The antennae are generally elongated and 

 projecting. The ocular pedicles are usually short. The 

 external jaw -feet are most frequently naiTOw, elongated, in the 

 form of palpi, and do not completely cover the other parts of 

 the mouth. The testa is more narrow, and more elongated 

 than that of the brachyura, and usually terminating in a point 

 at the middle of the forehead. For more ample details we 

 must refer to the memoir before mentioned, of MM. Audouin 

 and Milne Edwards. A character observed by them on the 

 lobster [Astacus marinus, Fab.) and which would be decisive, 

 if it applied to the other macroura, is, that besides the two 



