of a flagellum to the exopodite, presence or absence of a longitudinal groove 

 on the ventral surface of the ischium, and shape and breadth of merus. 



12. Chelipeds : presence or absence of a subterminal spine on the upper 

 border of the merus, and shape and gape of fiiigers. 



13. Legs: length and degree of robustness, and length of dactyli. 



My experience of the value of these points in dealing with Indian species 

 is as follows : — 



(1) Carapace : the shape and breadth vary with age and sex in the same 

 species ; the carapace tends to grow broader with age, especially in the male. 



The amount of dorsal convexity is more constant ; but Schenkel believes, 

 and I am inclined to agree with him, that residence in the cold, well-aerated 

 water of rapid mountain-streams tends to flatten the carapace, whereas 

 residence in the warmer, and therefore less oxygenated waters of the plains, 

 throws more work on the breathing-mechanism, and so tends to enlarge the 

 gill-chambers and thus to blow out the carapace. The inferences, then, are — 

 (1) that breadth of carapace alone is not a safe criterion of species; (2) nor 

 is convexity of carapace, m the case of forms living at greatly different 

 levels in the same basin. The "areolation" of the carapace, however, is 

 constant. 



(2) Cervical groove : as far as the Indian Potamonidce are concerned, the 

 course, the depth, the breadth, and the completeness or incompleteness of 

 the cervical groove are all of constant value, not only in discriminating, but 

 also in grouping the species, as will be noticed in the sequel. 



(3) Front : the relative breadth of the front changes throughout the life 

 of the individual, becoming less and less with age : so that this character is 

 fallacious ; the degree of declivity also varies in the same sj^ecies ; but the 

 shape of the front, in so far as it depends upon (1) the amount of convergence 

 of the sides, and (2) the sharpness of the supra-antennular fold, is not change- 

 able, as a rule. 



(4) Orbit : the presence or absence of a gap between the lower border and 

 the outer angle of the orbit seems to be a fixed "point " ; the shape of the orbit 

 does not vary, but the prominence of the outer orbital angle does. 



(5) Antero-lateral borders of carapace: their relative length (chord) is 

 pretty constant, but the arc may vary much ; their sculpture may get worn 

 with age, but is otherwise fairly safe ; the size of the lateral epibranchial tooth 

 is variable in many species. 



(6) Epigastric crests : the position of these, with regard to the front and 

 with regard to the post-orbital crests, is fairly constant in any species. 



(7) Post-orbital crests: the relation of these to the epigastric crests 

 (whether continuous with them or not, and whether overlapped by them or 

 not) and to the cervical groove (whether cleft by it or not) does not often 

 vary, nor does their relation to the epibranchial spine ; but the make of the 



