in certain African River- Crahs. 473 



surfaco an area closely ooverefl witli very sliort spine.", opposeJ 

 to an oval area of sinnlar spin^, VQ.vy short and not clavat^, 

 on tlie branch ioste;^ite. On the^ixie of the first and second 

 walking-legs and on the corresponding parts of the branchio- 

 stegal edge the spines and setaj, which are present in small 

 numbers, present no evidence o£ modifit;ation. 



In Polainon {Polamonaufe-i) lutidacft/lum, de Man, of which 

 I have examined the types and numerous other specimens, 

 the coxse of the chelipeds and of the first two pairs of walxing- 

 legs have on the u[)per surface an oval area whicii comes into 

 contact with the branchiostegite, but the spines with which 

 it is covered are very minute and scattered. On t!ie bran- 

 chiostegal edge there are. several rows of rather stout spines, 

 often with a slender setit'orm tip, which are most developed 

 over the bases of the three pairs of limbs just mentioned. In 

 this case it seems very unlikely that the parts can have a 

 stridulating function at all. 



All the species mentioned above belong to " Group " (of 

 which P. africanum is the type) in Miss Ratlibun's classifica- 

 tion of the subgenus Putamonaiites. The other West-Afiicau 

 species referred to this group, P, pelii (Herklots) and P. de- 

 cazei (A. M.-E.), are unknown to me, unless, indeed, the 

 specimens which I Iiave referred to P. pobeguini should 

 really bear the name of Herklots's sj^ecies. 1 regret especially 

 that I have had no opportunity of examining any of the 

 Oriental species which Miss Ilathbun places in the same 

 group. In no other members of the Potamonidae iiave I 

 found so far any indication of a similar apparatus. 



Although it remains to be demonstrated by observation of 

 the living animals that the structures here described are 

 actually used in the production of sound, it can har ly be 

 doubted that this will jn-ove to be the case at least in P. afri- 

 canum and P. pohguini. So far as I am aware, no stridu- 

 lating organs have hitherto been described in any members of 

 the Potamonidae. According to the summary given by 

 Oitmann (Hronn's ' Thierreich,' Crustacea, ii. p. 124.3), the 

 only Brachyura know'n to stridulate or possessing organs 

 supjjostd to have this function are species of Mututa, OziuSy 

 I'lulyonyclius, Ocypode, Macrophthahnus^ Sisanna, and some 

 other Grapsidic. in all these cases, however, and in the few 

 Crustacea of other groups (Palinuridte, Paguridea) which 

 have stridulating-organs, the arrangement is very different 

 from that described liere. The nniin portion of the apparatus 

 consists of ridges or granules on the surface of the exoskcdeton, 

 never of nioilitied spines, although the latter are commonly 

 found in the stridulatinji'-or'Tans of the Aiachuida. 



