UECAPODA. 161 



less vivid recL the colouring principle l)eing decomposed by boiling 

 water ; other combinations of this principle produce, in some species, 

 a very agreeable mixture of colours, that frequently border on blue 

 or green. 



The greater number of fossil Crustacea hitherto discovered belongs 

 to the order of the Decapoda. Among those of Europe, the oldest 

 approach to species now living in the vicinity of the tropics; the others, 

 or more modern ones, are closely allied with the living species of 

 Europe. The fossil Crustacea of the tropical regions, however, appear 

 to me to bear the closest similitude to several of those now found 

 there in a living state, a fact of much interest to the geologist, should 

 the study of the fossil shells of those countries, collected from the 

 deepest strata, furnisli a similar result. 



FAMILY I. 



BRACHYURA.— Kleistagnatha, Fab. 



Tail shorter than the trunk, without appendages or fins at the 

 extremity, and doubled under, in a state of rest, when it is received in 

 a fossula on the chest. Triangular in the males, and only furnished 

 at base with four or two appendages, in the form of horns, the supe- 

 rior of which are the largest, it becomes widened, and convex in the 

 females f , presenting beneath four pairs of double hairy filaments J, 

 destined to support the ova, and analogous to the sub-caudal natatory 

 feet of the Macroura, and others. 



The vulvae are two holes situated under the pectus, between the 

 third pair of feet. The antennae are small : each of the intermediate 

 ones, usually lodged in a fossula under the anterior edge of the shell, 



* The sections thus named are based on an ensemble of important anatomical 

 characters, and generally correspond to the Linnsean genera, and sometimes also to 

 those established by Fabricius in his earlier works. These families are more exteu- 

 tensivc than the sections thus named in my other writings : but if they be con- 

 sidered as first divisions of orders, and if what I have termed tribes be considered as 

 families, it will be seen that the method is essentially the same. There is, then, the 

 opinions of others to the contrary notwithstanding, no real discrepance in this 

 respect. On the same principle, the subgenera, with the exception of some whose 

 characters are too minute or too slightly marked, will become genera in a more 

 detailed and special system. 



t The apparent number of segments, which is usually seven, sometimes also varies 

 according to the sex; it is less in the females. Dr. Leach has made great use of 

 this consideration, which appears to us of but little importance, and opposed to the 

 natural order. 



X Several of these filaments exist in the males, but in a rudimental state. 

 VOL. III. M 



