1907.] CRUSTACEA OF THE THIRD TANGANYIKA EXPEDITION. 267 



now possess in this country a fairly complete series of this species 

 of both sexes. We have the possibility for the first time of com- 

 paring males with the female on which the genns was founded, 

 and this might in itself justify a redescription. But a comparison 

 between a large series of the form described as Lhnnothelphtisa 

 macidata and the specimens of Platythelpkusa armata (which I 

 was unable to make in 1899), has convinced me that the former 

 cannot be regarded as constituting a sejjarate genus, but falls into 

 place as a species of Platythelphusa. A third species of Platy- 

 thelpht(,sa is among the acquisitions of the last expedition, so that 

 the description which follows has been materially modified in 

 view of our much more extensive knowledge. 



Description. — Carapace almost quadrilateral ; antero-lateral 

 margins arcuated and armed with spines ; postero-lateral margins 

 but slightly arcuated., Front little deflexed, nearly straight. Post- 

 frontal crest distinct and perlated, but not extending to lateral 

 margins. Sub-orbital spine more or less distinct, in addition to 

 prominent inner sub-ocular tooth. A stout triangular process 

 descends from the external angle of the front, and may be pro- 

 duced into a small spine antero-distally. Antennse situated j)artly 

 behind and partly between this descending process of the front 

 and the inner sub-ocular tooth ; the distal segments thus escape 

 an appearance of displacement by the front. Merus of external 

 maxillipeds broader than long, the palp being attached to its 

 antero- internal angle ; ischium without longitudinal furrow. 

 Ambulatory legs somewhat compressed, the fourth leg consider- 

 ably shortei- than the others, and wdth its two terminal segments 

 broad and flattened. 



Remarks. — With the inclusion of Limnotlielpliusa, this genus 

 has lost nothing of its original distinctiveness. In the generic 

 description of Lir)inothelp]uisa, stress Avas laid on the simple nature 

 of the second antennal segment, which was undistorted by the 

 deflexed front. Although it was perhaps not very happily ex- 

 pressed, this character is just as typical of the other two species 

 we know from Tanganyika, and deserves emphasis accordingly. 

 In those genera, and subgenera where the front is considerably 

 depressed, the antennfe have the appearance, at least, of a lateral 

 displacement or distortion in consequence. All the' species of 

 Platytlielpliusa show the front little deflexed, but have a trian- 

 gular process descending from the external angle, which process, 

 however, passes to the front and side of the antenna, without 

 modifying its shape or direction. In order to make this distinctive 

 feature quite clear, the frontal region of all three species of Platy- 

 theljyhusa is figured, while corresponding figures are given of 

 certain well-known species of the subgenera of PotaiDion (Plate XVI. 

 figs. 2-7). 



Owing to the modification of the generic characters of Platy- 

 thelphusa and the suppression of the genus Limnothelphusa, 

 Miss Rathbun's key to the subfamily Potamoninaj * requires 



* Op. cit. t. vi. p. 245. 



