64 DISCOVERY REPORTS 



ages in any subsequent literature. In the endopods of the sixth to the eighth thoracic appendages of 

 H. falklandica, there are three distinct segments distal to the ' knee ', in addition to the dactylus which 

 is fused with the nail. The proximal two of these segments are short and the articulation by which 

 they are separted is oblique as seen in Fig. 9 K. On close examination of stained preparations, a strong 

 flexor muscle can be clearly seen inserted on the outer margin of the third segment and running 

 obliquely through the segment to the inner base of the claw. No definite musculature appears in 

 the other two segments, though in one preparation there seems to be a feeble muscle running unbroken 

 through them. Following the procedure adopted by Hansen (1925, p. 1 10) of determining the homo- 

 logy of the segments in the thoracic endopods of the Mysidacea by their musculature, it would seem 

 that here we have the very unusual arrangement of a distinct unsegmented propodus and it is the 

 carpus which has become secondarily divided by an oblique articulation. I have examined specimens 

 of H. fyllae from the west coast of Ireland and find that in them also the endopods of the posterior 

 three pairs of thoracic appendages have three segments distal to the ' knee ' precisely similar to those 

 of this species. The male genital organ is small and bulbous and faintly two-lobed at its distal end 

 (Fig. 9L). Pleopods of the male differ only in small details from those of H. fyllae; the fifth pair has 

 a slender sympod and the endopod in adult males is longer than the exopod (Fig. 9 M); of the female 

 first pair composed of a single segment; remaining four pairs two-segmented, becoming progressively 

 longer on the posterior somites ; fifth pair very long, more than one-fourth as long again as the long 

 sixth abdominal somite (Fig. 9 N). Uropods small and slender, only three-fourths as long as the telson ; 

 exopod with the proximal fourth of the outer margin of the proximal segment naked; remaining 

 three-fourths armed with about nine spines among the setae, the spines becoming progressively 

 longer posteriorly ; distal segment equal in length to the terminal spine of the outer margin ; endopod 

 slender and tapering, slightly longer than the exopod (Fig. 9 P). Telson very long and narrow, more 

 than twice as long as the sixth abdominal somite, nearly four times as long as broad at the base; 

 lateral margins almost parallel for over half their length, then narrowing in a slightly convex curve 

 to the narrowly-rounded, entire apex, armed on the distal four-fifths of their length with a dense row 

 of spines arranged in a series of large spines with small ones in the intervals between them. These 

 large spines are extremely long and slender, especially towards the apex of the telson and the margin 

 shows marked constrictions at their points of insertion; apex armed with two long spines with a single 

 shorter spine between them (Fig. 9 P). 



Length. Largest male 14 mm.; ovigerous females from 12-2 mm. to 15 mm. 



Remarks. This species shows a closer resemblance to the northern species H. fyllae than to the 

 antarctic species, H. antarctica. The size, proportions and armature of the antennae and the uropods 

 are very similar in the two species and differ from H. antarctica in bearing both spines and setae on 

 the outer margin of the scale and of the exopod. All the recorded specimens of H. antarctica have lost 

 the distal end of the telson, but from the proximal portions which are present it is evident that the 

 armature of this organ in H. falklandica more closely resembles H. antarctica than H. fyllae. The 

 spines of the lateral margins form a close series almost from the base to the narrowly rounded apex, 

 whereas in H. fyllae they are confined to the distal fourth of the margins. 



This new Antarctic species may readily be distinguished from the northern H. fyllae by the well- 

 marked ridges or keels which outline the hepatic area of the carapace and by the absence of an hepatic 

 spine on each side ; by the larger number of spines on the outer margin of the scale and of the exopod 

 of the uropod ; by the larger eyes forming a median eyepad with the antero-lateral angles produced 

 into bluntly rounded, diverging processes ; by the more developed pit-like structure on the proximal 

 dorsal surface of the antennular peduncle ; by the form of the slender fifth pair of pleopods in the 

 male with the exopod shorter than the endopod, whereas in H. fyllae it is considerably longer; and, 



