452 Mr. C. Spence Bate on the British Diastylidse. 



he described and figured in the ' Linnsean Transactions ' the ani- 

 mal in the collection at the British Museum. From the reduced 

 form of the members generally, many appear, on a careless exa- 

 mination, to be wanting ; hence it is that both Say and Montagu 

 mistook the character of their respective species. 



Taking each of the segments in succession, we observe that 

 that which supports the first pair of appendages in Crustacea 

 is strongly marked as an independent segment, both in Squilla 

 among the Stomapoda, and Palinurus among the Dccapoda ; 

 and that in the Decapoda when the segment itself is absent, 

 the eyes are still borne on projecting peduncles ; but in the whole 

 of this group not only is the segment absent, but the peduncles 

 themselves are wanting; and the eyes not only lose their jooc?oji:>^- 

 thalmic character, but the two are so closely associated as to 

 appear, as they probably are, but a single organ, and to general 

 observation fixed in the centre of the carapace, in which ano- 

 malous position they have been described by those who have 

 discovered the organ, except Kroyer. 



The second segment, or that which supports the first or in- 

 ternal pair of antennse, is closely associated with the third, or 

 that which bears the second or external pair of antennse ; the two 

 segments united together are attached to the next succeeding 

 by the posterior margin only, which is somewhat broader than 

 the anterior, the centre of which is slightly advanced, as if to 

 cover the organ of vision. The fourth segment, or that which 

 supports the mandibles, is developed posteriorly to the pre- 

 ceding, to which it is united by the entire width of the anterior 

 segment, but only at its posterior margin, for the lateral edges, 

 unlike what is found in the perfect Macroura, are free. The 

 lateral processes or wings of the mandibular segment extend 

 considerably forward on each side of the segments which bear 

 the antennae, and meet without uniting in front of the same. 

 This segment forms nearly the whole of the carapace, and sur- 

 rounds the anterior segments, which appear as a central patch 

 on the dorsal surface. 



The carapace is developed from the same segments as in the 

 perfect Macroura, but in this tribe covers only the first two or 

 three instead of all the segments of the thorax ; — obedient to a 

 law which I think has been made out in a previous paper (see 

 Ann. Nat. Hist. July 1855), that the anterior portion of the 

 carapace lessens in importance in relation to the posterior, and 

 that the whole decreases as the animal descends in the scale of 

 nervous centralization. Consequently the great buckler, which 

 in the Brachyura and Macroura protects the whole of the thoracic 

 portion of the animal, extends its defence only over the two an- 

 terior segments ; the last five are seen posterior to the carapace, 



