DECAPOD CRUSTACEA FROM MADAGASCAR. 



923 



of the mandibuLar palp is more than ordinary prominent." I 

 find that this statement applies not only to Hydrothelphusa, but 

 also, and in an even greater degree, to the other Madagascar 

 species that I have examined. In Hydrothelphusa the proximal 

 thickening forms a sharply marked ridge ; in Potamon madagas- 

 cariense (text-fig. 161, B), P. goiidoti, and P. methueni this ridge 

 overhangs a little as a free lobe. This evidently forms a transi- 

 tion to the bifid palp of the Gecarcinucinse, although it is suifi- 

 ciently far removed from the typical Gecarcinucine condition as 

 seen in Parathelphusa tridentata (text-fig. 161,0), for example, to 

 allow us to refer all the Madagascar species with confidence 

 to the subfamily Potamoninse, with which they also agree in 



Text-fio-. 161. 



Mandibular palp (denuded of setae) of three species of Potamonids. The lower 

 figures represent the palp as seen from below, the upper figures the terminal 

 segment as seen from in front. A. Potamon Jluviatile. B. P. madagascariense. 

 C. Parathelphusa tridentata. 



the shape of the male abdomen. It is possible that a similar 

 transitional form of the palp may yet be found in species of 

 Potamoninae from other parts of the world, but it is evident from 

 Alcock's remai-ks that it does not occur in any of the Indian 

 species ; and, in the absence of evidence as to the species of the 

 African and Malaysian Ptegions, it may be provisionally regarded 

 as suggesting specially close afiinity between the species found in 

 Madagascar. 



I do not think that much importance attaches to the fact that 

 in all the Madagascar species the palp consists of only two 



