SYSTEMATIC ACCOUNT 183 



where it meets, but does not fuse with, the corresponding plate of the opposite Hmb. The coxae of 

 the last three pairs of thoracic appendages have plates developed on both their inner and outer 

 margins, the latter extending on to the dorsal surface of the body where they are separated from their 

 respective terga by sutures. 



The pleon segments are all coalesced to form a single plate. The antennule consists of a peduncle of 

 three joints and a single flagellar joint. The antenna, which is setose, is considerably longer than the 

 antennule; it is built up of five peduncular joints and a single one to the flagellum; the distal portion 

 of each antenna is directed towards the mouth. The maxilliped has a five-jointed palp; the second 

 pair of thoracic appendages is short, setose and carried close to the mouthparts ; the third and fourth 

 pairs are long and setose, the remaining four pairs are all alike, and are ambulatory ; of these the fifth 

 pair is directed forwards, and the remaining three pairs (6-8) backwards. 



The first as well as the second pair of pleopods in the male is modified as a copulatory organ ; the 

 penis is single. The uropod has a large protopodite, with both endopod and exopod present but of 

 small size. 



The marsupiiim of the breeding female is large and formed from four pairs of lamellae carried on the 

 coxae of the second to the fifth pairs of thoracic appendages, its development is similar to that of 

 Idotea emarginata (Fabr.) (p. 154). 



This family is intermediate in position between the families Pseudidotheidae Ohlin and Astacil- 

 lidae Stebbing. The broad, dorso-ventrally flattened body of Xenarcturus g.n., recalls the body form 

 of the first family, although the breadth of the head and anterior thoracic somites is relatively greater. 

 The fusion of the head with the second thoracic somite is common to all three families; the condition 

 in Xenarcturus comes nearest to that of the astacillid genus Idarctimis Barnard; in this genus the head 

 is fused with the second thoracic somite, but sutures are distinct laterally, whereas in Xenarcturus the 

 pleura of the somite form free lobes. 



The fusion of the abdominal segments to form a pleotelson, and the presence of both exopod and 

 endopod on the uropod, are characters shared by members of all three families. 



The antennule of Xenarcturus has a single joint to the flagellum and this bears sensory setae, which 

 are more numerous in the male than in the female ; this character is also found amongst the Astacil- 

 lidae in the following genera : Antarcturus Zur Strassen, Microarcturus Nordenstam, Astacilla Cordiner 

 and NeastaciUa Tattersall; it is also a character of Pseudidothea Ohlin. 



The antenna of Xenarcturus differs from that in the other two families in being very setose and 

 directed towards the mouth, and in having a single joint to the flagellum; Pseudidothea has two, and 

 the Astacilla group three flagellar joints. The palp of the maxilliped is five-jointed like that of Pseudi- 

 dothea and some astacillids ; it agrees with Pseudarcturella chiltoni Tattersall, in the absence of coupling 

 hooks from the endite of the basipodite. 



The form of the thoracic appendages (2-8) is astacillid-like, but with one very important difference, 

 the fifth pair, though directed forwards, resembles the posterior three pairs; that is to say, it is strong 

 and ambulatory, not delicate and setose as in the astacillids. The second pair of thoracic appendages is 

 short and closely applied to the mouth parts, and, like that of the Arcturus group, has the dactylus 

 expanded and without a claw. The third and fourth pairs are long and setose. 



The modification of the exopod of the first pleopod as a copulatory organ in the male is also present 

 in the astacillid genera Antarcturus Zur Strassen, Dolichiscus Richardson, and Neoarcturus Barnard, 

 as well as in the family Pseudidotheidae. 



The diagnosis of the family Astacillidae, as given by Richardson (1905, P- 323). included as a 

 character 'the four anterior pairs of legs urdike the three posterior ones, not ambulatory, nor strictly 

 prehensile, directed forward, slender, ciliated', and it is therefore obvious that Xenarcturus, with 



