248 CEYLON PEAEL OYSTER REPORT. 



First peraeopods : side-plates rhomboidal, widening below. First joint as long as 

 tlit.* next 3 ; the 3rd longer than the 5th, which is longer and narrower than the 4th ; 

 the 4th 1ms a long and a short spine, and the 5th a group of strong spines on the 

 postero-distal margin. Dactylus slender, slightly curved. 



Second peraeopods with the side-plates much produced behind, otherwise like 

 the 1st (fig. 10. pp.-). 



Third peraeopods : 1st joint narrow-oblong, widening distally, about as long and 

 wide as the 4th, naked except a long spine on the produced end of the front margin, 

 the remaining joints spinous, the 5th about half as wide as the 4th. Dactylus 

 slender, straight (fig. 10. pp. 3 ). 



Fourth and 5th perseopods : 1st joints broadly oval, with two teeth on the lower 

 part of the hind margin ; 2nd very small ; 3rd much expanded behind, almost as 

 wide as the 1st and considerably wider than the 4th ; remaining joints as in the 

 3rd peraeopods. The three last pahs of peraeopods increase in length successively, 

 the last two pairs being much the strongest, the 3rd hardly reaching below the 

 1st joints of the 5th (fig. 10. pp. 5 ). 



The 1st and 2nd uropods are slender, the peduncles shorter than the equal 

 rami, all spinous; the 1st are much longer than the 2nd. Third uropods: 

 peduncle strong, cylindrical, with 2 or 3 teeth on the distal margin ; outer 

 ramus lamellar, spear-shaped ; inner apparently wanting in all the specimens 

 (fig. 10. up. 3 ). 



Telson convex above, broad, cleft less than half its length ; a tooth on the outer 

 side, 2 upright spines near the middle and a group by the tooth of each division 

 (fig. 10. t.). 



This curious genus was first described by Stebbing in the ' "Challenger" Eeport,' 

 from two imperfect specimens of which only one, a female, had a head, so that he was 

 not aware of the remarkable development of the upper antennae in the male. He 

 therefore placed the genus provisionally in the Pontoporeiidae. I have thought it 

 advisable to form a new family for it and the genus Argissa, which also has been 

 placed by Sars, with hesitation, in the same family and, like Platyischnopus, is 

 characterised by a similar development of the last peraeopods and of the upper 

 antennae of the male. The present species has much the same general appearance as 

 P. mirabilis, Steb., but differs in the sculpture of the last pleon segment, the absence 

 of eves, the proportions and armature of the antennae, form of telson, &c. 

 Subsequently ('Ann. and Mag. Nat. Hist.,' Ser. G, vol. xix., 1897, p. 1, plate v.) 

 Dr. C. Chilton described another species (P. neozelanicus) from Otago, also from an 

 imperfect female specimen, which differs in the gnathopods, and many other points 

 from the other two species. It is to be hoped that specimens of the present species 

 will be taken with perfect 3rd uropods ; from the appearance of the peduncle I should 

 expect the inner ramus to be long and easily detached as in the case of some of the 

 Gammaridse. 



