146 CLASS CRUSTACEA. 



lo 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, i^a5.,) 



Has the tail shorter than the trunk, without ajDpendages 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 filaments edged with 

 hair, destined to carry the eggs, and analogous to the natatory 

 subcaudal feet of the macrourous Crustacea and others. 



The vulvae have two holes, placed under the breast, be- 

 tween the feet of the third pair. Their antennae are small. 

 The 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.) 



