916 DR. W. T. CALMAX OX 



measurements given by other writers, owing to the frequent lack 

 of precise indication of the points between which measurements 

 liave been taken. In those given here " length " is measured 

 from the hinder edge of the carapace to the front in the median 

 line, i. e., to the bottom of the notch between the frontal lobes ; 

 "breadth" is the greatest breadth of the carapace wherever it 

 may be ; "exorbital width '' is the distance between the tips of 

 the exorbital teeth, and is therefore least accurate where these 

 points are blunt and indefinite ; " frontal width " is the most 

 difficult of all to find fixed points for, owing to the obliquity of 

 the sides of the front ; the measurements here given have been 

 taken between the innermost points of the orbits, ?". e., just above 

 the antennal flagella, when the carapace is viewed from in front. 

 Complete references to the earlier literature relating to the 

 species dealt with will be found in Miss Rathbun's monograph, to 

 which I am glad to acknowledge my great indebtedness. 



PoTAMON (Potamon) madagascariense A. Milne-Edwards. 



Potamon (^Potamon) madagascariensis A. M.-E., Rathbun, Nouv. 

 Arch. Mus. Paris, (4) vi. 1904, p. 264, pi. ix. fig. 7. 



I have ventured to group xaider this name a number of speci- 

 mens which dififer considerably among themselves, and of which 

 some at least would probably be regarded by other students as 

 representing distinct species. I have so grouped them, partly as 

 a confession of failure, for I feel no great confidence that they 

 really belong to a single species, although I have found no 

 satisfactoiy characters by which to distinguish them, and partly 

 in order to emphasize the fact that the various forms can be 

 arranged, in a way that seems to me significant, around the' 

 typical form of the species. They radiate outwards from this 

 typical form towards the other species of Potamonidse recorded 

 fi'om Madagascar, and while in some cases there remains enough 

 of a gap to justify the i^etention of the names given to these 

 other species, the relations are clear enough to suggest for the 

 Madagascar River-Crabs an autochthonous origin fi'om some 

 form reseinbling the typical P. madagasccn'iense. Miss Rathbun, 

 in her monograph, attributes a certain range of variation to 

 this species, and notes that one of the forms shows a transition 

 towards P. huinhloii. If the Adew here put forward be correct, 

 the i-ange of variation must be considerably extended, and 

 P. humhloti, P. graudidieri, and P. bomhetohense must be 

 included as varietal forms. 



The specimens examined appear to fall naturally into five 

 series as shown on the accompanying table of measurements. To 

 avoid prejudging the question of their systematic rank, I designate 

 these series by letters. 



