on a Crah from Christmas Idand. 547 



organisms, plant or animal, wliich it plucks from its sur- 

 roundings to " mask " its shell. The present ease seems to 

 deserve record on account of two circumstances, which suggest 

 that the association is here of a more intimate and constant 

 character : the first is the symmetrical way in which the 

 liydioid colony is disposed with regard to tlie crab; the 

 second is the fact that the type specimens of the species to 

 which the crab belongs, although coming from a distant 

 locality, are found on re-examination to carry colonies of a 

 closely similar, perhaps identical, species of hydroid. 



The crab belongs to the species described by Miers* from 

 the 'Challenger' collections as Medceus haswelli. It is a 

 female, about 6' 7 mm. in width across the carapace, and 

 was dredged at a depth of 45 fathoms off Flying Fish 

 Cove, Christmas Island. On comparing it with the three 

 imperfect type specimens from Twofold Bay, New South 

 "Wales (150 fathoms), the only difference that can be detected 

 is that the teeth of the antero-lateral margins, as well as the 

 granules on the surface of the body and clielipeds, are rather 

 less prominent and acute in the specimen from Cliristmas 

 Island. It is to be noted, however, that Miers's figure is not 

 q\nte accurate as regards the outline of the carapace, the 

 distance from the outer angle of the orbit to the first antero- 

 lateral tooth being considerably exaggerated. 



I cannot find that Miers's species has been recorded or 

 discussed by any author since its description. So far as can 

 be judged from the material examined, its affinity with the 

 genus Medceus is by no means clear. The characteristic 

 prolongation of the antero-lateral margin below the orbit is 

 represented, as Miers says, "merely by a very obscure line of 

 granules," and it is hard to find any other character which 

 might justify the assignment of the species to Medceus. On 

 the other hand, the proportions of the front and of the fronto- 

 orbital border suggest tiiat the species should be referred to 

 the subfamily Chlorodiinae of Alcock's classification f, and it 

 does not seem to differ in any important character from some 

 of the species included in the genus Xanthias { = Xanthodes') 

 in that subfamily. I propose, therefore, to refer to it as 

 Xanthias haswelli. 



The hj-droid is a gymnoblastic species, of which, in the 

 absence of gonophores, the systematic position cannot be 

 exactly determined. It has a creeping hydrorhiza, about 

 •07 mm. in width, closely adherent to the surface of the crab, 



* Rep. Brachyura 'Challenger,' p. 117, pi. xi. fig. 2 (1886). 

 t Journ. Asiatic Soc. Bengal, ixvii. pt. ii. p. 78 (1898). 



