246 "TERRA NOVA" EXPEDITION. 



Description. — Body of the usual Arcturid-like shape, geniculate at the junction 

 of the fourth and fifth free thoracic somites ; head with the anterior margin quite 

 straight and not excavated ; eyes prominent and bulging, pigment black ; head 

 with a pair of small cephalic conical horns ; last seven thoracic somites clearly 

 marked off, none of them elongate or differing in size markedly from the others, 

 without spines or tubercles ; side-plates present on all the thoracic somites, those 

 of the second somite not expanded to cover the oral area ; abdomen (pi. X, 

 fio-. 4) of two somites, but two other somites indicated laterally by grooves ; 

 terminal somite with a well-marked keel on each side of the median line, terminating 

 some way in front of the apex in small spines, the area between the keels flat, 

 and the lateral and terminal portions of the abdomen sloping sharply down 

 from the keels, so that in lateral view the pleon resembles the inverted keel of a 

 flat-bottomed boat. 



First antennae (pi. X, fig. 5) rather long and robust, extending almost to the end 

 of the third joint of the peduncle of the second antennae ; flagellum as long as the 

 peduncle, and with sensory setae arranged in regular pairs, ten in number, the whole 

 way along the lower margin. 



Second antennae (pi. X, fig. 6) robust, two and one-fifth times as long as the first ; 

 fourth joint of the peduncle one and a half times as long as the third, fifth joint one 

 and a quarter times as long as the fourth ; flagellum two-thirds as long as the fifth 

 joint of the peduncle, composed of two joints terminated by a strong spine, none of the 

 joints pectinate on the inner margin. 



Mouth-parts very much as in the genus Arcturella as figured by Sars, except 

 that I could not find any coupling hooks on the maxillipedes. 



Second thoracic limb (pi. X, fig. 7) with the inner distal corner of the carpus 

 produced into a short blunt spine-like process ; proximal portion of the dactylus linear 

 as in Arcturella, and not expanded as in Astacilla. 



Third to fifth thoracic limbs (pi. X, fig. 8) with the fourth joint not elongated, 

 fifth joint shorter than sixth, dactylus distinct though small. 



Sixth to eighth thoracic limbs (pi. X, fig. 9) robust, dactylus not bi-unguiculate 

 but with two spiniform setae on the inner margin near the tip. 



First pleopod of the male (pi. X, fig. 10) with a specially modified lobe on the 

 inside of the exopod. This lobe is expanded at the base and tapers to a point, 

 and is only slightly shorter than the rami of the pleopods. On its inner margin it 

 bears seven long plumose setae near the base, and the tip shows several transverse 

 thickenings of the chitin. 



Second pleopod in the male (pi. X, fig. 11) with the stylet on the endopod long 

 and pointed, nearly twice as long as the rami. 



Remarks. — I have discussed the affinities of this species under the genus. There 

 is only one specimen, a male measuring 4'5mm., but, judging from the pleopods at 

 least, sexually mature. 



