i84 DISCOVERY REPORTS 



its four posterior pairs of limbs alike, cannot be included in this family. The combination of this limb 

 arrangement with a dorso-ventrally flattened body removes it still further from the Astacillidae. 



The fact that the second, third and fourth pairs of thoracic limbs (first, second and third pereiopods) 

 are astacillid-like in form excludes the genus from the family Pseudidotheidae ; for these reasons the 

 new family Xenarcturidae has been formed to contain it. 



Genus Xenarcturis g.n. 

 Xenarcturus spinulosus, g.n., sp.n. (Plate VIII, fig. 3; Text-figs. 19-22) 



Occurrence. St. WS 237: 7. vii. 28, 46° 00' 00" S., 60° 05' 00" W., 150-256 m., i (?, i ? (breeding). St. WS 756: 

 10. X. 31, from 5o°53'S., 60° 00' W. to 50° 56-3' S., 59 56' W., 119 m., 2 S3- St. WS 782: 4. xii. 31, from 

 50° 30' S., 58° 19' W. to 50° 27' S., 58° 31' W., 141-146 m., I c?, I ?. 



The male holotype is from St. WS. 756, the female holotype from St. WS. 237. The genus has the 

 characters of the family. 



Description. The Discovery collections contain a single breeding and a single non-breeding female, 

 measuring 7-5 mm. in length by 3-5 mm. in greatest breadth, and four mature males of approximately 

 equal size, one of which measures 6-75 mm. in length and 3 mm. in greatest breadth. 



The form of the body may be seen from Plate VIII, fig. 3 ; the anterior margin of the head is broad, 

 and the breadth is gradually increased to the fifth thoracic somite, after which it decreases to the 

 extremity of the pleotelson, which ends in an acute point. The dorsal surface of the body is slightly 

 arched in the male, more so in the female; its surface is covered with scattered spinules. 



The shape of the head and second thoracic somite, and the position of the eyes are shown in Text- 

 fig, iga; the line of fusion of the head with the second thoracic somite is marked by a shallow groove; 

 the pleural portions of the somite are free and lie partly beneath the lateral margins of the head. 



The arrangement of the coxal joints and their plates has already been given in the diagnosis of the 

 family; the pleura of the second to the fifth thoracic somites are separated by grooves from their 

 respective terga; each tergum is raised in the middle, so that the animal when viewed from the side 

 has a slightly corrugated dorsal surface. The form of the pleotelson can be seen from Text-fig. 196. 



The antennule (Text-fig. 20a) is short and broad, the peduncle consists of three joints, the middle 

 one of which is the longest; the flagellum is a single, broad, rounded joint with sensory setae fringing 

 its tip and inner margin; these setae are more numerous in the male than in the female. 



The antenna (Text-fig. 20^) is very much longer than the antennule; it is bent sharply between the 

 third and fourth peduncular joints so that the flagellum is directed towards the mouth ; long setae 

 fringe the inner margins of the second, third and fourth joints. The flagellum consists of a single short 

 joint, tipped with three long setae. 



The form of the cutting edge of each mandible is shown in Text-fig. 20 c, d. 



The maxillula and maxilla (Text-fig. 20/, g) show the typical astacillid structure ; in the latter, the 

 broad fixed lobe has its rounded extremity fringed with plumose setae ; each of the two outer lobes 

 bears two plumose setae on its truncate distal extremity ; those of the outer lobe are the longer. The 

 palp of the maxilliped (Text-fig. 20 e) is five-jointed, the inner margin of each joint, except the first, 

 is fringed with long setae ; no coupling hooks are present on either the basipodite or its endite, but 

 the edges of these appear to interlock with those of the opposite maxilliped; as in other isopods, 

 a coxal lobe is developed on the coxal joint of the breeding female. 



The second thoracic appendage {first pereiopod) (Text-fig. 21a) is shorter and broader in proportion 

 than the succeeding appendages, and the dactylus is broad, with its rounded distal end fringed with 

 long setae; similar setae are also present on the inner margins of the ischium, merus, carpus and 



