CEUSTACEANS FROM THE TEOPICAL ATLAXTIC. 353 



segment. The boundary between the first and second segments is rather indistinct. 

 All the segments, except the first, have on the hind margin a strongly projecting 

 median tooth. The pleon is much narrower than the pergeon, but similarly armed 

 with a median tooth on each of the first three segments, of which the postero-latera] 

 angles appear to be acute. The fourth segment is short, with a thin semicircular 

 shield arising from near the front margin and covering the chief part of the segment. 

 The coalesced fifth and sixth segments are together not longer than the fourth. The 

 small telson is broader than long, distally truncate, not narrowed, with a setule on 

 either side of the centre of the distal margin; folded under is a thin curved lobe about 

 half the length of the telson. That the fold is natural and not accidental is evident 

 from the uninterrupted double marginal lines running round the sides and end of the 

 other portion. No eyes were perceived. 



The first antennae are longer than the head and perteon together. The long first 

 joint of the flagellum has serrate edges, with numerous hyaline bacilli along the whole 

 length and spines at intervals ; the terminal joint is quite small. 



The second antennae (in the female) are short, obscui'ely three-jointed, the first joint 

 being a broad tubercle, the other two joints linear. 



The mandibles are of the usual simple character, ending in narrow, finely denticulate 

 cutting plates. Of the other mouth-organs as much as could be made out is shown in 

 the figures ; they are not suggestive of anything exceptional, apart from the figure x, 

 which does not agree with anything hitherto described for this genus. Whether it 

 may be a part of the maxillipeds I have not been able to determine. 



The first gnathopods. The side-plates are bluntly pointed in front ; the second joint 

 has the edges almost parallel, with minute spinules along the front one ; the short third 

 joint has a terminal spine; the fourth joint is very little longer than the third ; it has 

 spinules on the hind margin and two apical spines ; the fifth joint is a little longer than 

 the sixth, the two together being longer than the second ; each carries a single spine 

 and a pair of spines at intervals on the hind margin ; the sixth joint is slightly curved ; 

 the finger is straight and slender, nearly half the length of the sixth joint. 



The second gnathopods nearly resemble the first, but the side-plates are larger, the 

 second, fifth, and sixth joints longer, and the finger seemingly shorter. The branchial 

 vesicles are broader than the second joint, and more than two- thirds as long. The 

 marsupial plates of the specimen are smaller than the branchial vesicles, and successively 

 smaller to the last, the fourth, pair. Of the five pairs of branchial vesicles the third is 

 the largest. 



The first and second perseopods are much longer than the gnathopods, with similar 

 but larger side-plates. The second joint is not wider and not greatly longer than the 

 fifth ; the fourth joint is rather longer than the sixth. There are no strong marginal 

 spines. The finger is minute, clasping between two sharp forward-directed teeth at 

 the apex of the hind margin of the sixth joint. In all the limbs the muscles are short, 



