THE TANGANYIKA PROBLEM. 283 



World forms, leaving only, in the section Thelphusinre, those typical of the Old 

 World, though occurring also in Australia. The principal points of resemblance to, 

 and difference from, the members of this group, which this Tanganyikan crab 

 presents, may be conveniently stated in tabular form. 



Points of resemblance to the Thelphusinre : — • 



(1). Presence of distinct post-frontal crest. 



(2). Conditions of sutures on carapace. 



(3). Form of external maxillipeds. 



(4). Character of chelipeds. 



(5). Spinuliferous condition of ambulatory dactyli. 



(6). Normal seven-jointed nature of abdomen. 



Points of difference from the Thelphusince : — 



( 1 ). Length of carapace more nearly equal to the breadth. 



(2). Carapace considerably less vaulted. 



(3). Antero-lateral margins relatively longer. 



(4). Greater breadth and less deflection of front, with larger size of orbits and 



eyes. 

 (5). Second joint of antenna not distorted by deflexed front. 

 (6). Spotted nature of test. 



Two genera only — Parathelphusa and Thelphusa — are included by Ortmann under 

 the heading Thelphusime. Of these, Parathelphusa was originally supposed to be 

 typically Indo-Malayan in distribution, but in 1887 A. Milne-Edwards included' 

 under this heading several forms originally described as Thelphusa from the African 

 continent. The genus Thelphusa is widely distributed over all parts of the Old 

 World. Comparison with the large number of specimens belonging to these two 

 genera in the collection of the British Museum showed that there are no forms 

 which would seem to be closely allied to Limnothelphusa, but so far as general 

 appearance goes, the specimens of Parathelphusa certainly agree most nearly. The 

 latter have a carapace more elongated in proportion, have larger spine-bearing 

 antero-lateral margins, and are considerably more flattened. The front, too, though 

 deflexed, is less so than in Thelphusa. On the other hand, however, in several of 

 the described species the abdomen of the male is of the so-called "hour-glass" 

 shape, while in all one spine only seems to be developed on the carpal joints of the 

 chelipeds, and the second antennal joint is distorted in the common manner. The 

 condition of the chelipeds is, however, in some species of Thelphusa strictly com- 

 parable with that of Limnothelphusa, so that in this respect we may consider the 

 new form as occupying a somewhat intermediate position between these two old- 

 established genera. 



Two other little-known genera, however, Hydrothelphusa and Pla/y/helphusa, must 

 apparently also be included in the group, though they are not mentioned by Ortmann. 

 Of these, the former, from the streams of Madagascar, was first described in 1872 

 by A. Milne-Edwards. The description, however, was very brief, and though he 

 has since given a further account, as well as a figure of the dorsal aspect, our 

 information is still unfortunately very incomplete. The front here, instead of being 

 deflexed, is said to be almost horizontal, while the carapace is onsiderably flattened 



