Female (figs. 1-14).

Overall length 8.2 mm-10.1 mm.

Cephalothorax subovate, width four-fifths length (2.06 mm-2.3 mm × 1.7 mm-1.85 mm), a little wider posteriorly than anteriorly; attached to second thoracic segment by a short neck, about three-quarters carapace width.

Nemesis lamna lamna Risso, 1826 from Isurus oxyrinchus. Female—fig. 1: dorsal view; fig. 2: anterior ventral view; fig. 3: first antenna; fig. 4: second antenna; fig. 5: mouth tube and mandibular palp, dorsolateral view; fig. 6: mandibular palp; fig. 7: maxilla; fig. 8: maxilliped; fig. 9: first pereiopod; fig. 10: second pereiopod; fig. 11: third pereiopod; fig. 12: fourth pereiopod; fig. 13: fifth pereiopod; fig. 14: caudal lamina.

Second thoracic segment subrectangular, length three-fifths width (1.5 mm-1.7 mm × 2.2 mm-2.6 mm), anterior width four-fifths posterior width, posterior margin sublinear, angles rounded; separated from third thoracic segment by a short neck which is three-quarters width of second thoracic segment.

Third thoracic segment subovate, length three-fifths width (1.2 mm-1.5 mm × 2.2 mm-2.6 mm), anterior angles broadly rounded, posterior angles rounded and swollen slightly posteriorly; the swelling of the posterior angles, of both second and third segments, is generally more pronounced in specimens from Carcharodon carcharias than in those from Isurus oxyrinchus; separated from the fourth thoracic segment by a short neck, two-thirds width of third segment.

Fourth thoracic segment similar in size and shape to third (1.3 mm-1.6 mm × 2.2 mm-2.6mm), but in some cases posterior angles are not swollen posteriorly to the same extent as in the third segment; separated from fifth segment by a neck, seven-tenths width of fourth segment, which is slightly longer than those between the three preceding segments.

Fifth thoracic segment subrectangular, length three-fifths width (1.2 mm-1.5 mm × 1.9 mm-2.5mm), narrowing to three-quarters this width posteriorly, anterior angles broadly rounded and swollen anteriorly, posterior margin sublinear, posterolateral angles with small acute swellings.

Genital segment subrectangular, a little wider than long (0.7 mm-0.95 mm × 0.8 mm-1.1 mm), with spermathecae which are dark brown in colour and about 0.4 mm in diameter borne laterally.

First abdominal segment subrectangular, length two-fifths width (0.2 mm × 0.4 mm-0.55 mm), narrowing slightly posteriorly.

Second abdominal segment a little wider than long (0.3 mm-0.4 mm × 0.3 mm-0.45 mm), lateral margins very slightly swollen, with caudal laminae borne laterally on posterior margin.

Caudal laminae, width one-third length (0.2 mm × 0.07 mm-0.08 mm), narrowing slightly at the base, posterior margin rounded, with four spines, the most medial very large the others small.

Egg strings long, 31 mm-43 mm, eggs uniseriate.

First antenna of thirteen segments, although the segmentation is not always distinct, ratio of lengths of segments—13:6:4:3:4:2:3:4:4:4:4:6:6; segments disc-shaped or subcylindrical; first segment, width half length, with two setae near outer distal angle; second segment as long as wide, with one seta near outer distal angle and one seta near outer proximal angle; third segment, length two-thirds width, with a seta near each of the distal angles and the outer proximal angle; fourth segment, length three-fifths width, without setae; fifth segment, length three-quarters width, with two setae near middle of outer margin and another seta on distal margin near outer distal angle; sixth segment, length half width, with one seta medially on outer margin; seventh segment, length two-thirds width, with one seta medially on outer margin; eighth to eleventh segments as long as wide, each with a single seta between middle of outer margin and outer distal angle; twelfth segment, width half length, with one seta near outer distal angle; thirteenth segment, width two-fifths length, with about nine setae on or near distal margin.

Second antenna of four segments, first, third and fourth segments subequal in length, second segment half this length; first segment subrectangular, width two-thirds length; second segment, subrectangular, rounded medially, as long as wide, with one seta on inner margin; third segment, basal width two-fifths length, narrowing page 4 to three-quarters this width distally; fourth segment a sharp claw, basal width two-fifths length, distal two-thirds rapidly narrowed, ending in a sharp point distally, swollen proximal portion with one spine near inner proximal angle, and a small projection near inner distal termination of swollen section, this projection bearing a further small spine.

Mouth tube 0.4 mm in length, basal width two-thirds length, narrowing gradually to half this width distally, distal margin irregularly rounded.

Mandibular palp biramous, each ramus of one segment, segments subequal in length, one-quarter length of mouth tube, width one-third length; outer ramus with one long seta and two very short setae distally, inner ramus with two very long setae, one plumose over the median third of its inner margin, and one short spine.

Maxilla of two segments; first segment four-fifths length of second, subrectangular, width half length; second segment basal width one-third length, distal third a process arising just proximal to a rounded margin which terminates the wider portion of the segment, the process narrowing to a point distally, distal margin of wider portion covered in small spines, and with a clump of long cilia just inside process, a second process, two-fifths length of first, arises near its base, is rounded distally and covered in small spines.

Maxilliped of two segments, subchelate; first segment basal width half length, narrowing rapidly and then more gradually to two-fifths this width distally, rounded distally, outer margin an entire curve, with a stout spine near inner proximal angle, and a small process on inner margin near point at which second segment closes against it; second segment basal width two-fifths length, narrowing gradually to a point distally, outer margin an entire curve, inner margin divided roughly into thirds by two small spines, the more distal one bearing an even smaller spine on its inner margin.

First pereiopod biramous; basipod of two segments, each segment subrectangular, length two-fifths width; exopod two segmented, first segment subrectangular, width two-fifths length, curving sharply medially near base, with a small spine on outer distal angle, a further small spine on distal margin near outer margin of second segment, a row of very short spines over distal two-fifths of outer margin, a row of long cilia over medial two-thirds of inner margin, and a group of very short spines on inner distal angle which is slightly rounded; second segment subrectangular, width three-quarters length, with three stout spines on distal margin, and a very small spine on inner distal angle; endopod basal segment subrectangular, length half width, with a very stout plumose seta on outer distal angle, and two well developed processes on distal margin each twice as long as segment, their basal width half their length, one narrowing to a more or less sharp point distally, with a few very small spines on its surface, the other, which is rounded distally and bears two short spines on the distal margin, may be the second segment of the endopod.

Second pereiopod biramous, each ramus of two segments; basipod of two segments, much larger than basipod of first pereiopod, basal segment slightly swollen laterally, with a short row of very small spines just medial to endopod; exopod two-fifths length of basipod, first segment subrectangular, outer distal angle swollen and rounded, outer margin slightly curved, with a stout spine on outer distal angle, a smaller spine on inner distal angle, a row of spines on distal quarter of outer margin and lateral half of distal margin, second exopod segment subsemiovate, proximal margin sublinear, two-thirds length of first segment, width two-thirds length, with five small spines on distal margin; endopod five-sixths length of exopod, first segment subrectangular, length two-thirds width, with a very stout spine on outer distal angle, page 5 another on inner margin, and a row of very small spines on outer part of distal margin, second segment subsemiovate, proximal margin sublinear, length subequal to basal width, narrowing slightly distally, with three spines on distal margin.

Third pereiopod similar to second except that basipod has a group of very small spines between the rami, and a small plumose seta just lateral to exopod.

Fourth pereiopod similar to second and third except that the basipod bears more very small spines between the rami, the exopod has a wide band of very small spines near outer margin, the second segment of the exopod has six small spines on its distal margin, and second segment of endopod has a wide band of very small spines near outer margin, two well developed spines on distal margin and three smaller spines just medial to these.

Fifth pereiopod small, subovate, length two-thirds width, with four spines on or near distal margin, the middle two being much larger than the outer and inner spines.

Male (figs. 15-26).

Overall length 5.9 mm-6.3 mm.

Cephalothorax shaped as in female, as wide as long (1.6 mm-1.8 mm × 1.4 mm-1.8 mm).

Second thoracic segment subrectangular, length three-fifths width (1.0 mm-1.2 mm × 1.8 mm-2.2 mm), anterior angles broadly rounded, posterior margin sublinear.

Third thoracic segment similar in shape to second but smaller (0.9 mm-1.1 mm × 1.6 rnm-2.0 mm).

Fourth thoracic segment similar in shape to second and third except that posterior angles are slightly more rounded, and the segment is even smaller than the third (0.8 mm-1.05 mm × 1.5 mm-1.8 mm).

Fifth thoracic segment much smaller than preceding segments, length half width (0.3 mm-0.4 mm × 0.5 mm-0.7 mm), widest posteriorly, narrowing to two-thirds this width anteriorly at junction with fourth thoracic segment, broad curved posterior margin articulating with genital segment.

Genital segment subovate, anterior and posterior margins sublinear, a little wider than long (0.9 mm-1.3 mm × 1.05 mm-1.3 mm).

First abdominal segment subrectangular, length half width (0.3 mm-0.4 mm × 0.5 mm-0.7 mm), narrowing slightly posteriorly, posterior angles rounded.

Second abdominal segment subrectangular, length half width (0.2 mm × 0.35 mm-0.5 mm), posterior angles slightly rounded.

Third abdominal segment length two-thirds width (0.25 mm-0.3 mm × 0.35 mm-0.4 mm), caudal laminae borne laterally on posterior margin.

Caudal laminae subrectangular, outer distal angle broadly rounded, inner distal angle slightly rounded, width two-thirds length (0.2 mm × 0.11 mm-0.15 mm), with two small spines on outer distal angle and a further two small spines on inner distal angle, the two middle spines being slightly larger than the outermost and innermost.

First antenna similar to that of female except in setation as follows: first segment has one seta on outer distal angle, second segment has one seta medially on outer margin and two on the outer distal angle, third segment has one seta near middle of outer margin, fourth segment has two setae on outer distal angle, fifth segment has one seta on outer margin and one on outer distal angle, fourth segment has two setae on middle of outer margin and one on outer distal angle, seventh, eighth and ninth segments have one seta each on outer distal angle, tenth and eleventh segments have page 6

Nemesis lamna lamna Risso, 1826 from Isurus oxyrinchus. Male—fig. 15: dorsal view; fig. 16: anterior, ventral view; fig. 17: first antenna; fig. 18: second antenna; fig. 19: mandible and mandibular palp; fig. 20: tip of maxilla; fig. 21: maxilliped; fig. 22: second pereiopod; fig. 23: third pereiopod; fig. 24: fourth pereiopod; fig. 25: fifth pereiopod; fig. 26: caudal lamina.

page 7 two setae each on outer distal angle, twelfth segment has one seta proximal to midpoint of outer margin, thirteenth segment has nine setae on or near distal margin which is more rounded than in female.

Second antenna, mouth tube, mandibular palp, maxilla and maxilliped as in female, except that inner margin of narrowing process on terminal segment of maxilla has most of inner margin slightly serrate.

First pereiopod as in female.

Second pereiopod as in female except that the segments of the rami are somewhat longer and narrower, the basipod has a less well developed lateral expansions and a greater number of small spines in the region of the rami, a number of small spines on the rami themselves are in the same area but increased in relative size, the large spine on the inner margin of endopod is missing, there are six spines on the distal margin of the second segment of the exopod.

Third periopod biramous, each ramus of two segments, first segment subrectangular, width two-thirds length, outer distal angle somewhat extended and bearing a stout spine, distal quarter of outer margin with a row of very small spines, and medial third of distal margin with a row of very small spines, second segment subovate, width half length, with three spines distally, the innermost longer than the other two; endopod subequal in length to exopod, first segment subrectangular, inner margin half length of outer, with large spines on outer distal angle and inner margin, second segment width two-fifths length, rounded distally, with one small spine and two setae on distal margin, the longer seta subequal in length to ramus, the outer seta half length of inner, and with a group of very small spines near middle of outer margin.

Fourth pereiopod biramous, each ramus of two segments, segments similar in shape to those of third pereiopod; first segment of exopod with a stout spine on outer distal angle, and a row of small spines on distal margin, second segment with five large spines and one small spine on distal margin; first segment of endopod with a few small spines associated with its base, a stout spine on outer distal angle, and a long seta a little shorter than ramus, on inner margin, second segment with distal setae like those of third pereiopod, and two smaller setae on inner margin near its midpoint.

Fifth periopod consisting of one large and two very small setae on a small swelling borne on fifth segment.