SYSTEMATIC REPORT 109 



particularly deep and well-marked; lateral margins sinuous as shown in Fig. 22 A; antero-lateral 

 corners produced into a right-angle from which a distinct ridge or keel runs obliquely backward to 

 fade away near the cervical sulcus ; anterior margins straight, produced forward to meet at an angle 

 of about 120 forming a very short rostral plate, which covers the posterior margins of the eyeplates 

 (Fig. 22 B). Abdomen strong and muscular with the sixth somite half as long again as the fifth 

 (Fig. 22 A). Antennule very short and stout, particularly in dorsal view; third segment as long as the 

 first and second combined (Fig. 22 A-C). Antenna/ scale with the naked outer margin relatively short, 

 terminating in a strong tooth, which extends to beyond the distal margin of the third segment of the 

 antennular peduncle; apex very long, occupying nearly one-half of the whole scale; small distal 

 suture present (Fig. 22 B, D). Antennal peduncle less than half as long as the scale; second and third 

 segments sub-equal, armed along their outer margins with a regular row of plumose setae ; no spine on 

 outer distal angle of the sympod. Eyeplates large and quadrangular with the ocular papilla well 

 developed and extending beyond the anterior margin of the eyeplate ; antero-lateral region densely 

 spinulose; the spinulation extends along the lateral region but becomes more sparse proximally; the 

 anterior margin on the inner side of the papilla is also sparsely spinulose, but the spinules give place 

 to dense, very minute bristles on and around the antero-median borders (Fig. 24A, B, J). Mandibular 

 palp particularly long and strong (Fig. 24A). Third to the eighth thoracic appendages with very large, 

 well-developed exopods; endopods long and slender with the two-segmented propodus separated 

 from the carpus by a very oblique articulation ; dactylus well-developed with a comparatively strong 

 curved nail (Fig. 24 A, E). Pleopods as in the other species of the genus in the male; those of the 

 female unusually long (Fig. 24A). Uropods with the exopods very long, nearly twice as long as the 

 telson and half as long again as the endopods ; a single long spine present near the inner distal border 

 of the statocyst (Fig. 24 F). Telson in the form of a long trapezium with lateral margins straight and 

 converging regularly to the broadly truncate apex; three times as wide at the base as at the apex; 

 hollowed from above in the form of a trowel, so that in dorsal view when attached to the animal it 

 appears much narrower than it actually is ; lateral margins armed along the distal three-fifths of their 

 length with a close, evenly spaced row of regularly graduated spines, which increase in size distally 

 and become of such a length that the long apical spines form the natural culmination of the graduated 

 series; apex truncate with a suspicion of emargination in the median region; armed with two pairs of 

 very long spines, of which the inner are slightly longer than the outer, and a pair of minute spines 

 flanking a median pair of long plumose setae. The gap in the regular sequence of spines running along 

 the margins and around the apex caused by the presence of these tiny spines is very striking and serves 

 as a ready means of distinguishing the species from the two other species of the genus (Fig. 24 G, H). 



Length of adult male, 20 mm.; of adult female, 22-2 mm. 



Remarks. In its general form and particularly in the form of the antennal scale and the telson, 

 A. obtusa closely resembles the two other species of the genus, A. crozetii ((W.-Suhm) G. O. Sars) 

 and A. ohlinii (W. M. Tattersall). But A. obtusa can at once be distinguished from these species by 

 its obtuse-angled anterior end of the carapace, because in both of them this margin forms an evenly 

 rounded curve with no trace of a median angle, and by the armature of the telson. In A. crozetii the 

 apex of the telson is armed with about 14 spines and a pair of median plumose setae. The posterior 

 margin of the telson is definitely emarginate so that the innermost pair of spines do not extend so far 

 back as those next them, although in fact they are slightly longer. The other apical spines are of about 

 equal length, the outermost ones a little shorter and merging into the graduated series of the lateral 

 margins (Sars, 1885 a, pi. xxxm, fig. 16). The effect of this arrangement of the apical spines in 

 A. crozetii is to make the telson appear to be much more sharply truncate than in the present 

 species. 



