ISOMETRINAE 175 



they differ too much from one another to be regarded as of one species. They must 

 therefore be known as Isometra lineata and /. angtistipinna, and brief descriptions of them 

 as such are given below. 



Mortensen (191 8) described /. vivipara from shallow water off the coasts of Uruguaya 

 and northern Argentina, from the Burdwood Bank, and from the eastern side of 

 Graham Land. 



In the present collection there are many specimens of /. vivipara from near the 

 Falklands and from the Burdwood Bank, and one specimen, differing slightly from them, 

 from the Bransfield Strait. There are small numbers of three new species, all viviparous, 

 one from the Shag Rocks, another from near Clarence Island in the South Shetlands, 

 the third, represented by specimens from Bismarck Strait, to the west of Graham Land, 

 from the Bransfield Strait and from the Ross Sea on the other side of the continent. 

 Two of the new species are very close to /. vivipara. 



The genus appears to be a well-marked natural assemblage. 



Isometra lineata (Carpenter) 



Antedon lineata Carpenter, 1888, p. 183, pi. xiii, figs. 4, 5. 

 Isometra angustipinna (part), Clark, 1908, pp. 133-4. 



The single specimen is much larger than that of /. angustipinna : Carpenter gives its 

 spread as "probably about 18 cm." 



The most complete of the remaining cirri is detached and probably lacks one or two 

 basal segments. It is of 30 segments. Stumps of other cirri remain on the centrodorsal 

 so that an almost complete description of a cirrus is possible. The first two segments 

 are short ; the third is longer, but broader than long. The fourth to about the tenth are 

 longer than broad, decreasing in length towards the tenth. The remainder are broader 

 than long and the more distal have a strong and characteristic dorsal spine the point of 

 which is sub-terminal ; it is well shown in Carpenter's figure. The spine is reduced on 

 the two or three segments before the penultimate. The ventral edge of the cirrus is 

 smooth. The opposing spine is strong and triangular; the terminal claw is large and 

 strongly bent. 



The shapes of the primibrachs and lower brachials are shown in Fig. 13 a; they have 

 sharp and nearly straight side-edges as in the other species of Isometra. The syzygial 

 pairs beyond the third are separated by one to four, usually two or three, brachials. 



Pi is of about 9 long slender segments and is ca. 5 mm. long; the first three segments 

 are attached by a web of tissue to the disc. Po and P3 are shorter, about 3-5 mm., of 

 8-9 segments ; the first two segments of P.^ are attached by a web of tissue to the arm. 

 The first genital pinnule is so far out as Pg or P, ; they are of about 9 segments and 

 ca. 5 mm. long. The third and fourth segments of the genital pinnules are slightly and 

 almost symmetrically expanded (Fig. 13 b)\ they are considerably longer than broad. 

 The specimen is a male. 



