1888.] MYRIOPODA, ETC. OF CHRISTMAS ISLAND. 563 



Two male (probably young) specimens taken under a log; in the 

 higher part of the island, about two hundred feet from the summit. 



In 1873 Prof. James Wood-Mason described a species of Land- 

 Crab which he raised to the rank of a new genus, Hylceocai-cinus, 

 on the strength of the separation of the front from the internal sub- 

 orbital lobe, two skeletal pieces which in Geocarcinus and Pelocar- 

 cinus are confluent. An additional diiferential character is the 

 paitiul concealment of the apical segments of the external maxillipede 

 behind the inner angle of the merus of that a[)peudage, these apical 

 segments being concealed in Geocarcinus, visible in Pelocarcinvs. 



Six years later Dr. De Man established a fresh genus, Linmocar- 

 einus, upon a specimen of a Land-Crab which differs mainly from 

 Hylceocarcinus in the entire visibility of the apical segments of the 

 external maxillipede and in the smaller space between the front and 

 tlie internal suborbital lobe. These, at all events, are the characters 

 upon which the genus was founded. 



Now there is a two-fold reason for regarding HyleBOcarcinus hutnii 

 and Limnocarcinus intermedius as referable to one genus. Firstly, 

 the degree of concealment of the distal segments of the external 

 maxillipede is a character which, in the allied genus Geocarcinus, is 

 subject to a considerable amount of variation, and therefore by 

 analogy is of no great value in the case of the species under discus- 

 sion. Secondly, HylcBOcarcinus humii is in one particular intermediate 

 between Limnocarcinus intermedius and the species described above ; 

 a species which, on account of the-freedom of the apical segments of 

 the external maxillipede, is undoubtedly referable to the genus Lim- 

 nocarcinus as defined by De Man. The above-mentioned particular 

 is found in the relations inter se of the front and the two suborbital 

 lobes. For in L. intermedius the distance between the front and the 

 mner lobe is said to be equal to half the distance between the 

 inner lobe and the outer lobe ; in H. humii the two distances are about 

 equal ; in H. natalis the distance between the front and the inner lobe 

 is about twice as great as the distance between the inner lobe and 

 the outer lobe. 



Now these facial features present in Geocarcinus an arrangement 

 which is at least as constant as the degree of concealment of the 

 apical segments of the external maxillipede. It is clear, therefore, 

 that the one character is of not less value as a sign of affinity than 

 the other. 



It is certainly true that, so far as the external maxillipede is con- 

 cerned, the relationship between L. intermedius and //. natalis is 

 greater than the relationship between L. intermedius and H. humii 

 or between //. natalis and //. humii. But the fact that in the otiier 

 character mentioned H. humii is intermediate between H. natalis and 

 L. intermedius appears to me to make it desirable either to consider 

 the three forms to be referable to but one genus, or to keep the names 

 Hylceocarcinus and Limnocarcinus for their respective species, humii 

 and intermedius, and to constitute yet a third genus for //. natalis. 

 But of the two courses it is assuredly more expedient to adopt the 



