288 Memoirs of the Indian Museum. [Vol. V, 



merus of the third bears a stout spine near the distal end of its lower border. All 

 the segments bear setae, most thickly on the carpi and propodi of the third and 

 fourth pairs and on the last two segments of the fifth. The proportional lengths of 

 the ischium, merus, carpus, propodus and dactylus in the third pair are as 16, 14, 

 9I, 6 and 2 : of the same segments in the fourth pair as 16, 23, 11, 10 and 2 : of the 

 same segments in the fifth pair as 22J, 15, 5J, 7^ and 5§ . Attached to the proximal 

 segments of the fourth pair of legs is a curious elongated plate (text-fig. 30^) which 

 extends forwards to the base of the third pair, lying close to the sternum. The 

 lateral margins of this plate are straight, a little convergent distally, and its apex is 

 deeply bifurcated. 



The abdominal somites are smooth above ; their pleura are rounded inferiorly. 

 The length of the sixth somite is about equal to that of the fifth. 



The telson is a little longer than the sixth somite and is shorter than both inner 

 and outer uropods ; it is slightly sulcate above and bears two pairs of small dorsal 

 spinules (not shown in text-fig. 30/). At the distal end of each lateral margin are 

 two pairs of spines and the apical portion between them is produced and at the 

 extremity rather sharply angled. The innermost pair of spines, which is much the 

 longest of the two, reaches to less than half the length of the produced median part. 

 The breadth at the level of the spines is a little more than one-third the total length. 

 The telson does not possess a feeble lateral prominence but on either side, situated in 

 the proximal third and on the ventral surface, are four oblique ridges, the three 

 anterior ones placed close together, the other rather more distant. The arrangement 

 of these ridges, which appear to be characteristic of the species, is shown in text-fig. 

 30g. In spirit specimens they have a nacreous lustre and perhaps represent a stridu- 

 lating organ, but I am unable to find that they possess transverse striae and there 

 does not appear to be any ridge on the basal segment of the uropods which could 

 be brought to bear upon them. 



The outer uropod (text-fig. 30/) is longer than the inner ; it is distally pointed, 

 setose on both margins and about three and a half times as long as broad. 



The largest specimen, an ovigerous female, is about 14 mm. in length. 



A synoptic key and references to the four hitherto known species of the genus 

 is supplied by de Man. 1 0. striaticauda is evidently a very close ally of 0. occiden- 

 talis, Ortmann 2 , from the mouth of the Tocantins River in Brazil. Ortmann's des- 

 cription is very brief and neither in it nor in the figures is there any indication of the 

 ridges found on the telson in the Indian species ; it is probable, therefore, that a 

 well-marked difference exists in this respect between the two forms. In 0. occidentales , 

 also, the eyes do not extend beyond the antennular peduncle and, according to the 

 figures 3 , the antennal scale is considerably longer than in 0. striaticauda, the basal 



1 De Man, Decap. ' Siboga' Exped., II, Alpheidae, p. 135 (1911). 



% Ortmann, Decap. Schizop. Plankton- Exp ed., p. 46, pi. iii, figs. 4, ä,a-z. (1893). 



3 The figures are perhaps not very reliable. That of the third leg at least is almost certainly erro- 



