12 FOSSIL MALACOSTRACOUS CRUSTACEA. 



the hand with a tubcrcidatcd crest on the u])per margin, and with three, four, or five 

 tubercles on the outer side ; the fingers are furnished with a few strong tubercidai- teeth. 

 The ambulatory legs are all smooth, rounded, shghtly compressed, and without any 

 armatm-e or projections of any kind. 



Such are the characters wliich exist in all the species at present known of this 

 genus ; and the characters of the species consist rather of variations in degree than in any 

 marked and distinct deviation from the normal form. 



The genus Xanthopsia, especially X. LeacJiii, is found in larger numbers than any 

 other Crustacean in the London Clay. It was founded by Professor M'Coy* upon the 

 well-known Cancer Lcachii of Desmarest. The latter distinguished naturalist had 

 received specimens, as he informs us, from Dr. Leach, to whom he dedicates the species 

 known to him, and which was most probably X. nodosa of JM'Coy. Desmarest considered 

 it as belonging to the genus Xantho of Leach, which he evidently looked upon as only 

 sub-generically distinguished from Cancer, A\hich latter name he consequently applied 

 to it. 



Professor MUne Edwardsf places it amongst those fossil species which appear to 

 belong to his genus Cancer ; but this mistake arose, doubtless, from the want of access 

 to more perfect specimens. The following observations of Professor JM'Coyj show 

 the view which that gentleman took of its affiuities. " It is nearer to Zantho by its 

 tubercvdatcd carapace, few tubercles on its latero-anterior margins, and position of the 

 external antennae at the inner canthi of the eyes, instead of bet^veen these and the front ; 

 but it differs in the great convexity of the carapace, and materially from both those genera 

 in hotlb sexes having seven separate joints in the tad, showing in this a closer relationship 

 to Pilumnus, from which, however, the strong nodidation of the hind part of the carapace, 

 and its oval, vaulted fonn, as Avell as the quadrilobed fr'ont and great extent of the gastric 

 region, distinguish it." The ground upon A\hich this supposed relation to Filumms 

 rests, even were it correct, woidd be of little comparative value. The number of miited 

 segments of the abdomen varies considerably even in the different species of certain 

 genera. In some, as in Leucosia, for instance, a perfectly natm-al and circumscribed 

 genus, some species have the whole of the joints of the abdomen united ; others the third 

 with the foiu-th, and the fifth with the sixth ; but it is very remarkable that Professor 

 M'Coy is entirely mistaken as to the structure of the male abdomen in Xanthopsis. 

 It is absolutely identical with that in Xantho. The third, fourth, and fifth segments 

 are as completely united in the former as in the latter form. In examining a considerable 

 number of specimens in M'hich the abdomen is perfect, I fomid them all in the condition 

 I have just mentioned. There is, in all cases, a slight transverse groove iudicating the 



* 'Annals of Nat. Hist.,' 1849, p. 162. 

 t 'Hist. nat. Jes Crust.,' vol. i, p. 380. 

 X Loc. cit., p. 103. 



