I. INTRODUCTION 



The Potamonidce, to give them their new patronymic, derived from an old 

 generic name that was exhumed about twelve years ago— the Thelpheusiens and 

 \helphimnw of Milne Edwards, the Telplmsinea of Dana, the Telphusidw of 

 other days— are one of the families of Cyclometopon or Cancroid Crabs. 



On pp. 27 and 28 of the first fascicle of the first part of- this catalogue 

 there will be found a scheme of classification of the crabs which shows the 

 position of the Cydometopa, as accepted and understood in this work ; and I 

 would further refer to a paper in the Journal of the Asiatic. Society of Bengal 

 for 1899, vol. Ixviii., part ii., pp. 470-473, for the scheme of classification of the 

 Cydometopa, to which I propose still to adhere. 



Both these schemes have recently been modified to a slight extent, and 

 incidentally the good old terms Cydometopa and Catometopa (to which at one 

 time the Potamonidw were referred) have disappeared; but modifications of 

 this kind seem to me to be, hke the manner of spelling Sam Weller's name, 

 entirely a question of taste and fancy. 



Of the five t-dm\\ies—Corystidw, Cancridw, Porttmida.', Xanthidw, and 

 Potamonid(e-— that constitute the tribe Cydometopa, the nearest connexions of 

 the Potamonidw seem, from every point of view, to be the Xanthidw. 



The Potamonida; have been variously arranged in subfamilies by different 

 authors, but I am not prepared to accept any of the subdivisions hitherto 

 proposed. This subject will be considered further on : for the present, it is 

 enough to say that the Potamonida' of the Oriental region can be grouped in 

 two subfamilies, namely— (1) Potamoninoi, and (2) Gecarcinucinw or Para- 

 telphusina?. The definitions of these subfamilies, and of the genera and 

 subgenera assembled under each, will be found in the sequel. It is necessary, 

 first, to say something of the habits of the Potamonida' ; of their geographical 

 distribution, with special reference to the Oriental region; and of certain 

 things that make both the identification and the grouping of the species of this 

 family particularly difficult. As it will be necessary, in dealing with these 

 matters, to refer to certain species by name, the initial proceeding must be to 

 present a list of the Indian species included in the family. This list can not be 



