262 DR. W. A. CUNNINGTOK ON THE BRACHYUROUS [Mar. 5, 



POTAMON (POTAMONAUTES) Sp. ? 



Reference has already been made to the fact that it is often 

 difficult to distinguish between closely similar species of this 

 genus. Many of the characters enumerated by the systematic 

 writers on this gi'oup, while in certain cases affording a ready 

 means of separating species, are in other cases of little or no 

 value. A careful consideration of many of the described species 

 and the examination of an extensive series of specimens lead to 

 the undoubted conclusion that some species have been established 

 without sufficient justification. "While a species or even a genus 

 may be established on the strength of a single specimen, if its 

 characters are sufficiently unusual, it is necessary to be very 

 cautious in doing anything of the kind within the limits of this 

 subgenus (Fotamonautes) in particular. Yet there are several 

 species of Potamonautes based upon single specimens, and even 

 upon specimens which were recognised as being immature. We 

 do not know a great deal about the modifications in form produced 

 by advancing age, but we do know, from the examination of any 

 extensive series, that they are considerable. The jDroportion 

 of length of carapace to its breadth, with the relative proportions 

 of the front, the orbits, and the fronto-oi'bital width — all of 

 them characters employed by the systematist — are imdoubtedly 

 dependent upon the age and growth of the individuals. 



But more than this. A natural hesitation to lay stress on 

 exevj little difference in form is only emphasized by the result 

 of Schenkel's * investigation of the species Potamon {Potamo- 

 nautes) celehensis de Man. He is able to distinguish some six 

 local varieties, in addition to the forma tyinca of de Man. These 

 difier from one another in colour, in the shape of legs and carapace, 

 in the extent of the sculpturing, and especially as regards the 

 degree to which the carapace is vaulted and the bi"anchial regions 

 dilated. But, as Schenkel points out, these features are precisely 

 those which would be aflected by a difference of surroundings. 

 He finds the Crabs with the flattest carapace come from mountain- 

 streams, where of course the water is pure and well oxygenated. 

 The converse holds equally good, the gill-chambers in other 

 varieties being inflated \w proportion to the sluggish or muddy 

 nature of the' river inhabited, and it is obvious that the sculpturing 

 is in a great measure an expression of the degree of inflation of 

 the carapace. Again, it is pointed out that the arnount of the 

 food-supply must have a powerful influence on growth : thus with 

 two Crabs of similar size, that from water poor in nourishment 

 will be far older than the othei', and so not strictly comparable 

 with it. 



As Schenkel considered it desirable to retain a single species, 

 but to constitute a number of varieties in this case, this discussion 

 of the facts is very suggestive in a general sense. Under such 

 circumstances, some observers would not even separate into 



* Verb, natui-f. Ges. Basel, Bd. xiii. 1902, p. 528. 



