44 CEYLON PEARL OYSTER REPORT. 



two were identical, and suspected that in any case both would have to be transferred 

 to Arcturides, the genus of the third. He recognised that in the latter event the 

 family name would have to be changed. It would rather inconveniently become 

 Arcturididse. Another family, likewise intermediate between the Idoteidse and 

 Astacillidae, is now required. This is remarkable for the negative character in which 

 it agrees with the Amphipoda Caprellidea, a common result being no doubt referable 

 not to any close tie of consanguinity, but to the simple fact that in each instance 

 nature has enabled a species to get rid of limbs for which it had no further use. For 

 the genera Antarcturus and Pseudoprion, zur Strassen, see 'Zool. Anzeiger,' vol. 25, 

 p. G86 ; vol. 26, p. 31 ; 1902-03. 



Family : AMESOPODID^E, nov. 



Body not geniculate, but antennae, mouth organs, first gnathopods, and appendages 

 of pleon nearly as in the Astacillidae. Second gnathopods ambulatory, not setose, not 

 fully jointed. First and second pairs of perseopods unrepresented, except by the 

 marsupial plates in the female. 



Amesopous, n. gen. 



To the characters of the family none can be added for generic distinction, so long as 

 the family contains but a single genus. It may be noticed that the first segment of 

 the peraeon is coalesced with the head, that all the segments of the pleon are fused 

 into one, and that the wrist and hand of the first gnathopods are fringed with 

 conspicuously trifid setae. 



The name is framed to express the default of the median pairs of legs. In Cleantis, 

 Dana, a genus of the Idoteidae, which in general facies makes some approach to the 

 present, the second perseopods are the smallest of the limbs. In Arcturides the head 

 is only incompletely separated from the first perseon segment, but Ohlin finds this 

 fusion complete in his Astacilla falclandica, and nearly so in A. magellanica. 



Amesopous richardsonae, n. sp. Plate XI. (A). 



Head united to first peraeon segment without apparent suture, rostral point minute, 

 lateral lobes produced about to the end of the first joint of the upper antennae. 

 Body in male narrowly cylindrical, the limbless segments of the perseon the smallest, 

 but in the ovigerous female the cephalothorax widens distally, and the second, third, 

 and fourth peraeon segments which carry large marsupial plates are much wider, the 

 third and fourth being also considerably longer than any of the three following 

 segments. The pleon is narrowly ovoid, narrowly rounded at the apex. In lateral 

 view the dorsal outline is corrugated, and the female has a pair of dorso-lateral 

 tubercles on each of the second, third, and fourth peraeon segments. 



The eyes are dark, laterally protuberant just below the frontal lobes of the head. 

 The facets are small and numerous. 



The first antennae reach some way along the third joint of the second pair which 



