1899.] CEUSTACBAN TEOM LAKE TANGANYIKA. 701 



proportion, have larger spine -bearing antero-lateral margins, and 

 are considerably more flattened. The front, too, though deflexed^, 

 is less so than in Thelijhusa. 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 Thel- 

 jphusa strictly comparable with that of Limnoihel^jJiusa, 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 

 Platythelphusa, must also, I suppose, 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 considerably 

 flattened and nearly quadrilateral. Only a single tooth, however, 

 is present on the antero-lateral margin, in addition to that at the 

 outer angle of the orbit. With this the description of Platy- 

 tlielphusa ^, which actually comes from Lake Tanganyika, agrees in 

 the main, but the antero-lateral margins are, in contradistinction, 

 multi-dentate. Several figures of this form are given, but they 

 are not, unfortunately, all one could wish. The figure of the 

 antennae suggests that we are dealing with a simple undistorted 

 condition of the joints, such as I have seen nowhere else but in 

 Limnotlielphusa, but the right and left antennae do not even agree 

 one with another, according to the drawing. The fourth pair of 

 walking-legs presents a peculiarity in being rather short, while 

 the terminal joints are somewhat flattened and expanded, pre- 

 sumably for swimming purposes. The male of this form is 

 unknown, so that it is to be hoped that Mr. Moore, during his 

 present expedition to Tanganyika, will obtain further material, 

 and so aid in clearing up this unsatisfactory state of our knowledge. 

 Of the mode of life of either of these forms little or nothing can 

 be learnt from the paper, which fact renders it still more difficult 



^ Milne-Edwards's descriptiou of the genus Farathelphum (see Ann. Sci. Nat. 

 iii., Zool. t. 20) is exceedingly brieP, and as regards the deflection of the front 

 certainly misleading. Using this definition, one might readily conclude that 

 Limnothelphusa conies under it, though an actual comjiarison shows that the 

 resemblance is by no means exact. 



^ Would it not be more satisfactory to keep these forms separate by con- 

 stituting two new genera or sub-genera, this extremely prominent difference in 

 shape of the male abdomen being made the basis of separation, as indeed has 

 been done by Wood-Mason in his note on the genus (see Ann. & Mag. Nat. 

 Hist. 1876, p. 122) ? 



3 Ann. Sci. Nat. v., Zool. t. 15. * Ann. Sci. Nat. vii., Zool. t. 4. 



