181 

 The branchial formula is as follows : — 



Somites and 



Total 5 + (7ep.) 10 . 4 = 19 4 7 ep. 



Key to the species of Arctus of the Indian Oligobenthos. 



I. Thoracic sterna smooth, 4th pair of thoracic legs the longest — a good 



deal longer than the carapace ... ... ... ... A. orientalis. 



II. Thoracic sterna with a median tubercle : 2nd pair of thoracic legs 



longer than the 4th pair and shorter than the carapace ... A. rubens. 



106. ArctllS Orientalis, Spence Bate. 



Arctus orientalis, Spence Bate, Challenger Crnst. Macrura, p. 68, pi. ix., fig. 4 : Alcook and Anderson, Journ. 

 As. Soc. Bengal, Vol. LXIII, 1894, pt. 2, p. 165. 



Carapace, measured in the middle line, about as long as the abdomen 

 without the telson, its length about equal to its greatest breadth at the antero- 

 lateral angles : its lateral margins are serrulate and are cut into three lobes 



an orbital, an hepatic, and a branchial : it is dorsally traversed by 3 longitudinal 

 carinas— a median, and one running from either inner-orbital angle — which are 

 coarsely crenulate but carry no salient teeth : its surface carries some small 

 tubercles, the most conspicuous of which define the cardiac region laterally : its 

 posterior border, which is separated from the rest of the carapace by a deep 

 fissure, is very slightly notched in the middle line. 



Thoracic sternum deeply concave, without any median series of tubercles. 



On the abdominal terga and pleura there are some small squamiform tuber- 

 cles, in two transverse series on the terga, and in a marginal series on the pleura. 

 The 2nd-5th terga are longitudinally carinate, the carinas being distinct but not 

 salient, and being neither produced nor notched posteriorly. 



More than half the 3rd joint of the antennular peduncle projects beyond 

 the antenna?. 



The proximal (outer) antennal squame has both its edges serrated, and is 

 traversed by a single oblique midrib : the distal lobe has its free margin cut into 

 7 teeth, of which 2 are on the inner margin. 



