PART 5 A MONOGRAPH OF THE EXISTING CRIXOIDS 627 



ISOMETRA GRAMINEA John* 



Isomctra graminea JOHN, Discovery Reports, vol. 18, 1938, p. 132 (in key), pp. 185-187 (stations; 

 description); text fig. 17, p. 186; pi. 5, figs. 5, 6. (Not /. graminea John, Rep. B.A.N.Z. Ant- 

 arctic Res. Exped., ser. B, vol. 4, pt. 6, 1939, p. 206, which is here distinguished as I. johanni 

 sp. nov.). 



Diagnostic features. The cirri are XXVI-XLIII, 28-41; P 2 is slightly larger than 

 PI, with usually one more segment, 8-10 as opposed to 7-9 in PI ; the first genital pinnule 

 is P 4 or P 5 and in the female usually only the third and fourth segments are expanded. 



Description (from John, 1938). A female specimen from the Bransfield Strait 

 has the arms 48 mm. long. They taper evenly from the base. At the first syzygy the 

 width is 1.3 mm. From IBr, to the second syzygy is 8.8 mm. 



The centrodorsal is a rounded cone with its ventral edge produced into very slight 

 interradial corners. Most specimens have a bare dorsal pole. The cirrus sockets are 

 closely set in alternating rows, which may be irregular. 



The cirri are XXVI-XLIII, 28-41. There is some variation in the specimens from 

 different localities, those from the Ross Sea have the first three or sometimes four 

 segments considerably stouter than the succeeding segments, a feature not shown by 

 the specimens from the Graham Land area. In general the fourth or fifth to the eighth 

 or ninth segments are distinctly longer than broad and may be slightly constricted 

 in the middle. The following segments decrease in length and the distal ones are 

 produced into a moderate or strong, rounded dorsal spine. In the longer cirri these may 

 be keel-like. 



The radials vary in length. In the smaller specimens they are longer in the 

 midline than the IBr! but are shorter in larger specimens. The distal edge is wider 

 than the proximal and is concave. The IBr t of adjacent rays are not in contact 

 with one another. They are incised by the posterior projections of the IBr 2 (axil- 

 laries), which may form slight shoulders with them. In one of the syntypes the IBr 

 series and the lower brachials are raised into a keellike ridge in the midline. These 

 ossicles in all the specimens are relatively smaller than those of /. vivipara. Their 

 side edges are sharp and straight. The brachials are similar in shape to those of 

 7. vivipara, also the positions of the syzygies are alike. The distal edges of the outer 

 brachials are raised into moderately strong spines. 



The first three pinnules increase slightly in size from P! to P 3 . P! has 7 to 9 

 segments and is from 3 to 5 mm. long. P 2 may have one more segment than the 

 corresponding P, or the same number and is usually about 0.5 mm. longer. In the 

 Bransfield Strait specimen P, has 9 segments and measures 3.5 mm., while P 2 has 

 10 segments and measures 4 mm. P 3 has 9 to 12 segments in most specimens and 

 measures up to 7 mm. The lowest segments of these pinnules are attached by a web 

 of tissue to the disk or sides of the arms. 



The first genital pinnule is P 4 or P 6 and has 10 to 12 segments; the last is P 13 

 to P 16 where there are about 15 segments and the length is 7 to 9 mm. The species 

 is brood-protecting and the third and fourth segments of the genital pinnules of the 

 females are enormously expanded, mainly on the aboral side, to cover the ovary and 

 brood pouch. Also the fifth segment on the middle genital pinnules may be slightly 

 expanded. Similarly the third and following segments of the genital pinnules of the 



* See also table 15 (p. 620), scatter diagram (p. 835), and Addenda (p. 837) under 1963. 

 556-62267 *1 



