1 1*6 
CLASS CRUSTACEA. 
to our climates. But the fossil Crustacea of the tropical 
regions appear to me to have the greatest relations with many 
of those found there at the present day, in the living state ; a 
fact which would be interesting to geology, if the study of the 
fossil shells of those countries, and collected from the deepest 
strata, should present us with a similar result. 
The first family, or that of 
Brachyurous Decapods, (Kleistagnatha, Fab.,) 
Has the tail shorter than the trunk, without appendages or 
fins at its extremity, and folding underneath, in a state of re- 
pose, to lodge itself in a fosset of the chest. Triangular in the 
males, and furnished only at its base with four or two ap- 
pendages, of which the upper are larger, in the form of horns, 
it becomes round, broader, and gibbous in the females. Its 
under part presents four pair of double fdaments edged with 
hair, destined to carry the eggs, and analogous to the natatory 
subcaudal feet of the macrourous Crustacea and others. 
The vulvas have two holes, placed under the breast, be- 
tween the feet of the third pair. Their antennm are small, 
dhe intermediate usually lodged in a fosset under the anterior 
edge of the testa, terminate, each, by two very short threads. 
The ocular pedicles are generally longer than those of the 
macrourous decapods. The auricular tube is almost always 
petrous. The first pair of feet is terminated by a claw. The 
gills are disposed on a single range, in the form of pyramidal 
tonguelets, composed of a multitude of little leaves, piled one 
upon the other, in a direction parallel to the axis. The jaw- 
feet are generally shorter and broader than in the other de- 
capods. The two external ones form a sort of lip. Their 
nervous system again, differs from that of the macroura. (See 
the generalities of the decapods.) 
