702 MB. W. A, CXJNNINGTON ON A NEW [May 16, 



to effect a coinparisou with the other kuowii types. Troui this 

 unfortunate lack of information then, though it is difficult to 

 determine the exact relations which these two genera should hear 

 to those more fully known, there can, I think, be no doubt that 

 both are wholly distinct from Lintnotlieljihusa. Several features go 

 to prove tliis : among them the feeble development of the post- 

 frontal crest in both Hydrotheliilmsa and Flatythelphiisa ; but 

 perhaps the most conspicuous difference is the greater breadth of 

 the front and larger size of the orbits and eyes in Limnothelphusa. 

 in the existence of but one marginal tooth, in association with an 

 almost horizontal coudition of the front, we have in Hydrothelphusa 

 a rather anomalous feature — a combination, as we shall have 

 reason to see, of a specialized with a primitive character. Thus, 

 while in respect to the condition of the front this form would appear 

 to be closely allied to riatythelphusa and Limnothelphusa, as regards 

 the nature of the antero-lateral margins, its affinities are rather 

 with the genus Thelphusa itself. 



Plati/thelphusa, in the possession of a httle deflexed front, of 

 perhaps an undistorted antenna, and of a multi-dentate margin to 

 the carapace, stands clearly related to the only other form which 

 combines these primitive characteristics — this new genus Limno- 

 thelphusa. More than this, in the present state of our knowledge, 

 it is impossible to say, and which of the two last-mentioned genera 

 may be fairly considered the more primitive further information 

 alone will enable us to judge. 



Of the manner in which this form attained its present distri- 

 bution in Lake Tanganyika there are t«o possible views. Either 

 from a land Thelphusan it has become converted gradually into a 

 wholly aquatic type, or it may have entered the lake n)ore or less 

 directSly from the sea, in those early times when, as has been 

 suggested S the connection between them was far more close than 

 at pi-esent. It is generally accepted that the Land-Crabs have 

 descended from ancestors with a littoral habit, so that there would 

 be no dii'ect objection to the supposition that this creature has 

 merely retained its primitive aquatic character, rather than re- 

 gained it after adaptation to a terrestrial mode of existence. We 

 can only come to a conclusion on this head by estimating Jiow far 

 the general structure of the animal suggests simplicity on the one 

 hand, or, on the other, specialization. The arched or vaulted 

 condition of the branchial regions of the carapace in Thelphusa is 

 evidently a specialization in connection with aerial respiration. 

 That such prominent vaulting does not here exist is not surprising, 

 but though it is perhaps conceivable that this character, once 

 attained, might be lost again on change of environment, it is^ I 

 think, more probable that such a condition was never reached by 

 Limnothelphusa. Again, as regards the less prominent deflection 

 of the front in the latter, the condition appears rather primitive 

 than secondarily acquired ; while the simple nature of the second 

 antennal joint, as compared with that of Thelphusa, which so 



» Q. J. M. S. vol. xli. p. 303. 



