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- 
rn ea of Fabricius, the first subgenus of the next family, and 
thus make the passage from 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, Fab., 
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 laminas, 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 antenna) are generally elongated and 
projecting. The ocular pedicles are usually short. The 
external jaw-feet are most frequently narrow, 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 marinas , Fab.) and which would be decisive, 
if it applied to the other macroura, is, that besides the two 
