788 W. T. CALMAN. 



The species of the genus Ortinannia (foi-raerly known as 

 Afcyoida) differ from those of Caridina chieHy in the fact 

 that the carpus of the second pair resembles that of the first 

 pair ('JV^xt-fig'. -i, B' , B"), being short and broad, with its distal 

 niargin excavated and articulating with the propodus at its 

 lower corner. It is to be noted that these characters are not 

 equally well tnaiked in all the species referred to Ortmannia; 

 in some the second carpus is still, as in Caridina, somewhat 

 longer than the first, and the excavation of its distal margin 

 is shallow (as in Text-fig. 4, B") ; in other species the carpus is 

 nearly similar in the two pairs and so deeply excavated as to 

 assume an almost crescentic form (as in Text-fig. 6, B' , B"). 

 Associated with this excavation of the carpus is a shifting 

 (already begun in Caridina) of the carpo-propodal articulation 

 from the proximal end to the lower border of the propodus. 

 Further, while in some species the chelae themselves are quite 

 similar to those of Caridina, in others the "palm" is much 

 shortened, oi", in other words, the articulation of the movable 

 finger is carried backwards towards the base of the propodus. 



These modifications lead towards the conditions found in 

 the genus Atya, which includes the largest and most highly 

 specialised members of the family. In these the two pairs of 

 chelipeds (Text-fig. 3, A', A") are quite similar, and the carpus 

 is reduced by the excavation of its distal border to a narrow 

 crescent, with the lower limb of which the propodus articulates. 

 The propodus itself assumes a form unlike that of any other 

 Decapod; the backward shifting of the articulation of the 

 dactylus has been carried so far that the palm has entirely 

 disappeared, and the chela is composed of two similar parts, 

 hinged together at one end, like the legs of a pair of 

 compasses. 



Although, within each of the genera, there is some varia- 

 tion in the degree to which these characters are developed, 

 this variation is so far discontinuous that all the known 

 species could, prior to Bouvier's researches, be referred 

 without much difficulty to one or other of the genera. If it 

 be objected that such apparently trivial differences should 



