892 THE VOYAGE OF H.M.S. CHALLENGER. 



The ophthalmopoda in Hectarthropus compressus reach to the extremity of the 

 rostrum, whereas in Hectarthropus exilis they are nearly half longer; the frontal 

 extremity of the lateral margin of the carapace is strongly serrate in Hectarthropus 

 exilis, whereas in Hectarthropus compressus, though the serrature is present, the toothing 

 is but feebly marked, while the tooth on the post-cardiac region of the carapace is more 

 strongly pronounced, and those on each side of the posterior margin of the fifth somite 

 are less so. 



Observations. — These measurements are important distinctions, but still they may 

 be dependent on the variability of growth ; but until that is determined, it is desirable 

 that the form should be noticed in its relation to other specimens. 



Hectarthropus expansus, n. sp. (PI. CXLIV. figs. 4, 5). 



Carapace dorsally as broad as long, anteriorly produced to a sharp, smooth rostrum, 

 and armed on the frontal crest with a short tooth ; supraorbital tooth large, and pro- 

 jecting anteriorly ; second antennal tooth large. 



Pleon dorsally smooth in the median line ; second somite armed with a tooth on the 

 anterior margin of the coxal plate ; fifth somite armed with a tooth at the posterior 

 margin on each side of the dorsal surface. 



Telson as long as the sixth somite. 



Habitat. — October 23, 1874; off Basilan Strait, Philippine Islands; at the surface. 

 Associated with the preceding species. 



The carapace is expanded dorsally, and not laterally compressed as in the preceding 

 species. It is anteriorly produced to a smooth, straight rostrum, that is about one-third 

 the length of the carapace, and is dorsally armed on the frontal crest with a short stout 

 tooth. From the base of the rostrum the frontal margin obliquely recedes to the supra- 

 orbital tooth, which projects above the orbit, the outer portion of which is seen as an 

 excavation between the preceding and the orbital tooth that forms the outer canthus. 

 The second antennal tooth stands as a large, oblique, anteriorly directed tooth, whence 

 the frontal margin descends, projecting anteriorly to the fronto-lateral angle, posterior to 

 which the lateral margin is serrate, with a series of four or five teeth; behind this 

 serration the margin is smooth to the posterior extremity. 



