470 Dr. W. T. Caiman on a Stridulating-organ 



clevolnpment appears to be, without doubt, tliat described by 

 A. Miine-Edwards, and more fully by Dr. de Man and Miss 

 Rathhun, as Potamon [Potamonautes] afri'canum'^. 1 have 

 examined three specimens, all males. The largest specimen, 

 from which the following description is mainly taken, 

 measures 80 mm. across the carapace. The stridulating- 

 organ is formed by groups of modified spines on the upper 

 surface of the coxje of the first and second pairs of walking- 

 legs and on parts of the free brancliiostegal edge of the 

 carapace immediately opposed to them. The upper surface of 

 eachcoxal segment (fig. 2) is strongly convex, and the modified 

 spines occupy its posterior part, which curves downwards 

 towards the ridge separating the upper from the posteriur 

 surface. Ou the tirst leg the patch of spines measures about 



Fig. 1. 



Fig. 1. Potamoji {Potainoncmtes) africanum, male, from the right side 

 (natural size). 1 and 2, the first two pairs of walking-legs, on 

 the coxal segments of which are seen the patches of modified 

 spines opposite the lobes on the branchiostegal edge of the 

 carapace. 



3*5 X G"5 mm., and is conspicuous to the naked eye because of 

 the dark brown colour of the closely-set spines. On the second 

 leg the patch is less sharply defined and less conspicuous, 

 owing to the fact that the spines are smaller and more widely 

 spaced. In both cases the spines increase in size from the 

 anterior edge of the patch, where they merge into the scattered 

 setie of the general surface of the limb^ to the posterior edge, 



* Descriptions and full synonymy of all the species referred to will be 

 found in Miss Rathbun's monograph, " Les Crabes d'eau douce/' Nouv. 

 Arch. Mus. Paris, (4^ ser.) vols, vi.-viii. (1904-1906). 



