DKCAPODA. 
161 
less vivid red, the colouring principle being 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. 
Tlie 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, furnish 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 I’est, 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 sxipe- 
rior of which are the largest, it becomes widened, and convex in the 
females f, presenting beneath four pairs of double hairy filaments 
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 ai'e small : each of the intermediate 
ones, usually lodged in a fossula xinder 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 exten- 
tensive 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. 
-f- 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 importanee, and opposed to the 
natural order. 
J Several of these filaments exist in the m,ales, but in a rudiraental state. 
VOL. TIT. M 
