DBCAPODA. 



vivid rrd, the colouring principle being decomposed by boiling 

 w.iter ; other combinations of this principle produce, in some spec 

 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 hoar the closest similitude to several of those now found 

 then- 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, Fait. 



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 ba>e with four or two appendages, in the form of horns, the supe- 

 rior of which are the largest, it 1 ccomes widened, and convex in the 

 tt'inalesf, presenting beneath four pairs of double hairy filaments J, 

 .ned to support the ova, and analogous to the sub-caudal natatory 

 t. t 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 



j, 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 Lianaean penera, and sometimes also to 

 li>hrd by Fubriciu> in his rarlirr work*. These families are more cxtcn- 

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

 -i<l. rt.l UN fir-t ilhUinns of orders, nnd if what I ha\c termed tribes be considered as 

 families it \vill br 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 thr same prnriple, the snhirriiera, uith the exception of some whose 

 character* 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 \aries 

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

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

 nil order. 



t Several of thene filaments cxht in the male?, but in a rudimen'al <taf. 

 VOL. in. M 



