4-72 Dr. W. T. Caiman on a Stridulat'ing-organ 



two squnre-cut lobes, the distal margins of which nearly tonch 

 tiie surfaces of the coxre (fig. 1). Each lobe is tliickened and 

 is defined in front and behind by a notch, running upwardsas 

 a short groove on the surface of the carapace. On the lower 

 surface of each lobe (fig. 3) is a group of very peculiarly modi- 

 fied spines, unequal in size and dark brown in colour. Tiiese 

 spines (fig. 4), of which there are ten on the anterior and eight 

 on the posterior lolJ^in the specimen examined, are cylindrical 

 for a short distance from the base, then dilating into a stout 

 fusiform bulb and terminating in a sharp point. The dilated 

 part is hollow and its cavity is filled with air. In the basal 

 part the cavity contracts to a narrow canal. 



When tlie coxa of either leg is rotated backwards and 

 forwards about the vertical axis of the joint connecting it 

 with the trunk, a hissing or creaking sound is produced in 

 spirit-specimens, especially if the carapace be lightly pressed 

 down so as to bring the branchiostegal spines into contact 

 with those on the coxte. 



The species most nearly approaching P. africanum in the 

 degree of development of this organ is one from the Gama- 

 roons which I identify, not without hesitation, as Potamon 

 [Putamonautes] poheguini, Rathbun. In this species the 

 chelipeds, as well as the first and second walking-legs, share 

 in the formation of the stridulatitig-apparatus, which is 

 similarly developed in both sexes. The coxal spines on all 

 three pairs of limbs are much smaller and less closely set 

 than in P. africanum, and the areas which they occupy are 

 less sharply defined. The free edge of the branchiostegite is 

 slightly produced downwards between each of the successive 

 pairs of legs, but does not present sharply defined lobes as in 

 P. africanum. Viewed from below (fig. 5), the branchiostegal 

 edge above the coxa of the cheliped is seen to carry a patch 

 of numerous and closely-set spines, of which only the larger 

 are distinctly clavate and hollow. The groups of branchio- 

 stegal spines corresponding to the first and second walking- 

 legs resemble more nearly those of P. africanum, but the 

 spines are much more numerous and present a transition from 

 small, stout, but not clavate spines on the inner side to large, 

 clavate, hollow spines on the outer side. None of the spines 

 have the dark brown colour which renders conspicuous those 

 of P. africanum. 



In Potamon (Pofamonautes) floweri, de Man (of which I 

 have examined the type specimen), in P. auhryi (M.-E.), and 

 in another closely allied species from Fernando Po (to whicli 

 I do not venture to give a name), the apparatus is only repre- 

 sented on the chelipeds, of which the coxse have on the upper 



