XANTHOPSIS. 13 



junction, but in no case is there any separation between the segments in question, 

 whilst the first from the second, the second from the third, the fifth from the sixth, and 

 the sixth from the seventh, are, in every instance, umnistakeably disjoined. The 

 only possible relation of Xanthopsia to Pilumnus is thus removed, and its affinity to 

 Xantho strengthened. 



There is, however, another affinity, and that, as it appears to me, a very near one, 

 which has not been alluded to by any former writer. I mean that indicated by many 

 points of its structure to the genus Carpilius. In the general form of the carapace, 

 and particularly the extraordinary curve from the front to the back, as well as a 

 considerable arch from side to side, it differs essentially and strikingly from Xantho, 

 and perfectly resembles Carpilius. The incurved and quadrifid front, which is found in 

 every species of both genera, is no less obvious ; and in the arrangement and form 

 of the external and internal antennas, the orbits, and the external footjaws, the structure 

 approximates much more nearly the corresponding organs in Carpilius than in Xantho 

 or any other existing genus ; and in the simple rounded form of the ambulatory legs 

 it possesses the only important character which distinguishes Carpilius from Platypodia 

 {Cancer of M. Edwards). In fact, with the exception of the remarkable nodosities 

 upon the different regions of the carapace, the existence of four more or less obvious 

 tubercles on the latero-anterior margin, and the slight difference in the male abdomen — 

 which in Carpilius has only the fourth and fifth segments united, whilst in Xanthopsis 

 the third is also united to these — there are scarcely any striking characters separating 

 the two genera. It is thus extremely probable that in any complete natural arrangement 

 of the Canceridce, the genus Xanthopsis would occupy an intermediate and osculant 

 position between Xantho and Carpilius. 



The principal sources of my information on tliis genus, as in all the Crustacea 

 of the London Clay, have been the British Museum, the splendid collections of 

 Mr. Bowerbank and Mr. Wetherell, with a considerable number in the collection of 

 Mr. Prestwich, and in my own. I have also had the opportunity of examining the 

 specimens in the Cambridge Museum, which contains the identical specimens described 

 by Professor M'Coy. These altogether amount, probably, to two or three hundred 

 of the present genus. They of course vary greatly in their condition, and in the 

 preservation of the different organs ; but the whole have enabled me to ascertain 

 and describe with great certainty and exactitude most of the minuter parts of struc- 

 ture on which the generic character mainly depends, and which are essential to the 

 true understanding of the affinities. Thus the orbits, the eyes, the whole antennal region, 

 with the basal portion of the external and internal antennae, and at least two joints of the 

 filament of the former, the external footjaws, the carapace with its regional prominences, 

 the margin with its tubercles or spines, the abdomen of each sex, the entire chelee 

 and several joints of the ambulatory feet, have all been before me in a sufficiently perfect 

 condition to enable me to ascertain their structure, and to restore, as it were, to its original 



