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RUWENZOEI EXPEDITION REPORTS. 



5. CRUSTACEA. 



By W. T. Caljian, D.8c., F.Z.S. 



Eeoeived and read November 17, 1908. 



[Text-figures 9-12.] 



The river-crab of Ruwenzori (I find no reason to suppose that more than one species 

 is represented in the collection) belongs to one of the most puzzling and imperfectly 

 known sections of a very difficult genus. 



All the specimens diff'er in some degree from the type-specimens of Potamon johnstoni 

 (Miers) from Kilimanjaro with which I have compared them, but they appear to 

 approach more closely to that species than to any of the others hitherto described, 

 and I do not consider that the diff'erences justify their separation under a new specific 

 name. 



Only the acquisition of large series of river-crabs from all parts of Africa will enable 

 the species inhabiting that continent to be properly defined, and I may take this 

 opportunity to remind collectors that river-crabs from any part of Africa (and, indeed, 

 from most parts of the world) will be very gladly received at the Natural History 

 Museum. 



Potamon (Potamonautes) johnstoni (Miers). 



Thelplmsa depressa Krauss vav. johnstoni Miers, Proc. Zool. Soc. 1885, p. 237. 

 Potamon (Potamonautes) johnstoni Rathbun, Nouv. Arch. Mus. Paris, (4) vii. pp. 160 & 

 170 (1905). 



Twelve specimens were collected by the Ruwenzori Expedition, and seven specimens 

 from the same region, presented by Sir H. H. Johnston in 1901, are in the Natural 

 History Museum. 



The smallest specimen measures 12-5 mm. and the largest 3o-5 mm. in length of 

 carapace. The breadth-ratio of the carapace varies from 1-32 to r48, being on the 

 whole less in the smaller specimens. The surface of the carapace is moderately flat, 

 except in its anterior third, where it is convex antero-posteriorly. The inter-regional 

 grooves are strongly marked, especially the central part of the " cervical " groove and 

 the transverse branchial grooves or posterior branches of the cervical ; the latter con- 

 strict the branchial regions so that the postero-lateral margin of the carapace is concave 

 or almost notched. The anterior or lateral limbs of the cervical groove die out almost 

 immediately in front of their junction with the posterior branches. The inner branchial 



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