Cynstacea. 247 



Akcturus. 



This genus received a large number of additions from the 

 'Challenger' collections (2); and, later, Mr, Benedict (3) added 

 six species to the genus, and gave a synopsis of the whole. 

 Another species has since been described by Miss Eicliardson (2G), 

 and the three described below make a total of twenty-six species. 

 Two other species, described by Haswell, from New Zealand in 1882 

 (9), have not been noticed by the authors cited. 



Aecturus polaeis. 

 (PI. XXXIV., fig. 2, and PI. XXXV.) 



Specific character. Spines dorso-lateral. Pleon with two promi- 

 nent terminal spines, a large lateral spine on each of the three 

 distinguishable segments, the remainder somewhat irregularly covered 

 with short spines, two of which, about the middle of the lateral 

 margin, are more prominent than the rest. 



The largest specimens secured by the ' Southern Cross ' Expedition 

 measure, exclusive of antennae or spines, some 37 mm. in length, 

 but the average size is about 5 mm., or thereabouts, smaller. 



The second antennae are a little shorter than the body. 



The mid-dorsal area is smooth ; but dorso-laterally there are 

 spines which are more profninent in the anterior thoracic region, and 

 less numerous than elsewhere. 



Cephalon. Anterior margin incurved. A prominent spine pro- 

 jects outwards, and somewhat forwards, to protect the eye, which is 

 large. There is a small spine in front of this organ. 



The third thoracic segment is the largest, the hinder ones tapering 

 gradually to the pleon. Laterally each segment bears numerous 

 spines of varying size. The spines are most conspicuous on the three 

 anterior segments, and each of these has a large spine more dorsally 

 situated than elsewhere. 



Pleon nearly as long as the posterior live thoracic segments. 



The first three segments are distinguishable, and bear short spines 

 both dorsally and laterally. A lateral spine is especially prominent 

 on the third segment, and the remainder of the pleon is covered with 

 small spines of approximately uniform size ; but two of them, about 

 the middle of the lateral margin, are more prominent than the rest. 

 The mid dorsal line is devoid of spines. The extremity is rounded, 

 and bears two very large and somewhat divergent terminal spines. 



