190 DISCOVERY REPORTS 



distal margins. The costals are three to four times as broad as long. They are not incised 

 by the axillaries : they are bent back beneath its posterior projection. In older specimens 

 they are bent back farther than in the figure, so far that their mid-line, seen in profile, is 

 in line with the base of the centrodorsal. The costals of younger specimens are not in 

 opposition, those of older specimens are. The axillary is considerably broader than long 

 in younger specimens but nearly as long as broad in the older in which the posterior 

 projection is more strongly developed. The outer edges of the costals and axillaries are 

 straight and sharp. 



Syzygies are numerous. The normal positions of the first three are 3 + 4, 9 + 10, 

 15 + 16, but there may be small irregularities. Beyond the third syzygy the syzygial 

 pairs are separated by two or three, or exceptionally one or four to six, brachials. 



The outer edges of the lower brachials are sharp and their sides flat. The exterior 

 edges of the first brachials are longer than the inner ; the inner are in partial or complete 

 contact. The first brachial is only slightly incised by the second in younger specimens, 

 more deeply in older specimens. The outer edge of the second brachial may be twice 

 as long as the inner. The inner edge of the first syzygial pair is longer than the outer. 

 The brachials between the first and second syzygies are considerably broader than long 

 and longer on one side than the other — on alternate sides in successive brachials. An 

 obvious feature of older specimens is the strong moulding of these and a few succeeding 

 brachials. Their surfaces are not flat : the inner distal corner of the fourth brachial (an 

 epizygal) is raised, in common with the inner proximal corner of the fifth brachial, into 

 a prominence ; the outer distal corner of the fifth brachial and the contiguous corner of 

 the sixth brachial are raised into a similar prominence; and so on. The surfaces of these 

 lower brachials are quite smooth. 



From the second to the fourth syzygies the brachials become more wedge-shaped. 

 The distal edges of the outermost may be slightly everted and produced into fine spines 

 (Fig. 18 c). The shape of the brachials then passes to that of a rounded triangle ; the base 

 slopes across the arm and the rounded apex, produced into a strongly overlapping edge 

 armed with strong spines, is directed towards the side from which the pinnule arises. 

 The zigzag appearance may be stronger than that shown in Fig. 18 d. In the distal 

 brachials the shape again becomes quadrangular; the distal edges remain strongly 

 everted and spiny (Fig. 18 e). 



The pinnules gradually increase in length from Pj to the distal pinnules ; they become 

 shorter again towards the tip of the arm. Pj is short and stout, 4-5 mm. long and of 

 10, II or 12 smooth rounded segments which are longer than broad (Fig. 18/). P., is 

 similar and of the same number of segments but slightly longer and stouter, 5-8 mm. 

 long. P3 is similar, of 11-13 segments and slightly longer. P4 may be similar to P3 or 

 it may be the first genital pinnule. As in the other species of Isometra the oral pinnules 

 are not entirely free. Pj is attached to the disk by its first four or five segments, and the 

 first four or five segments of Pg and Pg are connected by webs of tissue with the under 

 side of the arm. 



This species, like the other Antarctic Isometras, is viviparous. The first genital 



