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Show 702 MR. W. A. CUNN1NGTON ON A NEW [May 16, to effect a comparison with the other known types. From this unfortunate lack of information then, though it is difficult to determine the exact relations which these two genera should bear to those more fully known, there can, I think, be no doubt that both are wholly distinct from Limnothelphusa. Several features go to prove this : among them the feeble development of the postfrontal crest in both Hydrothelphusa and Platythelphusa ; but perhaps the most conspicuous difference is the greater breadth of the front and larger size of the orbits and eyes iu Limnothelphusa. In the existence of but one marginal tooth, in association with an almost horizontal condition 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 Platythelphusa and Limnothelphusa, as regards the nature of the antero-lateral margins, its affinities are rather with the genus Thelphusa itself. Platythelphusa, in the possession of a h'ttle 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 Limnothelphusa. 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 distribution in Lake Tanganyika there are two possible views. Either from a land Thelphusan it has become converted gradually into a wholly aquatic type, or it may have entered the lake more or less directly from the sea, in those early times when, as has been suggested *•, the connection between them was far more close than at present. It is generally accepted that the Land-Crabs have descended from ancestors with a littoral habit, so that there would be no direct objection to the supposition that this creature has merely retained its primitive aquatic character, rather than regained, it after adaptation to a terrestrial mode of existence. We can only come to a conclusion on this head by estimating how 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 iu 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 ou 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 antenual joint, as compared with that of Thelphusa, which so 1 Q. J. M. S. vol. xii. p. 303. |
