618 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



exceptionally (Isometra hordea) up to 75; the cirrus segments are short, the longest 

 being barely longer than the median width, rarely up to twice as long as wide; the 

 division series and lower brachials are always closely apposed with straight sides, -the 

 proximal pinnules are relatively short, usually tapering evenly from a wide base, with fairly 

 stout segments not conspicuously expanded and thorny at their distal ends but on the 

 contrary often tapering; P, has 5 to 17 segments and may be longer or shorter than P 2 

 which has 6 to 14 segments; the genital pinnules are particularly characteristic, the 

 mature ones with at least two markedly expanded segments towards their bases form- 

 ing a brood pouch in the female for the developing young, the species being viviparous; 

 a similar expansion occurs in the males but is longer, involving more segments, and 

 less abrupt; the first genital pinnule varies in position from P 2 to P 7 , but the position, 

 within a limit of two or three pinnules, appears to be characteristic of the species. 



Geographical range. Known from the South Atlantic off the southern tip of 

 Brazil southwards to the Falkland-Magellanic region, the Shag Rocks near South 

 Georgia, the South Shetlands, Graham Land and the shores of Antarctica. 



Bathy metrical range. Known from 79 to 1097 meters. 



Remarks [by A.M.C.]. In the manuscript Mr. Clark had included Isometra in 

 the subfamily Bathymetrinae. He gave no explanation of his reason for this change, 

 presumably made about 1923 when the major part of this work appears to have been 

 written, and as far as I can see his only published intimation of this move is in his 

 report of the crinoids of the Australasian Antarctic Expedition (1937), where he lists 

 Isometra vimpara under the heading Bathymetrinae. 



It is possible that he was influenced by Mortensen's description (1918) of Phrixo- 

 metra, a genus of Bathymetrinae with a pouchlike marsupium on each genital pinnule 

 of the female, although this does not involve an expansion of the pinnule segments 

 themselves as there is in Isometra. However, hi my opinion, this analogy does not 

 constitute a sufficient link to justify amalgamation of the subfamilies Isometrinae and 

 Bathymetrinae. Not only do the species of Isometra have this very distinctive modi- 

 fication of the segments of the genital pinnules but also the relatively short, stout, 

 tapering oral pinnules of Isometra are very different from the slender ones of the Bathy- 

 metrinae with their markedly elongate outer segments. The cirri too of Isometra 

 are much more stout with relatively shorter segments than those of most species of 

 the Bathymetrinae. In fact, in comparing the subfamilies of Antedoniclae, I found 

 the Isometrinae as clearly marked off from the Bathymetrinae as the other subfamilies, 

 if not more so. The new species of Isometra collected by the Discovery Investigations and 

 B.A.N.Z.A.R.E. show a constancy of characters linking them closely with the older species. 



There is evidence that Mr. Clark himself had second thoughts about merging the 

 Isometrinae and the Bathymetrinae, as in the list of species for the Contents of this 

 work, which he made subsequent to Dr. Dilwyn John's Discovery Report (1938), the 

 Isometrinae is included separately. 



Genus ISOMETRA A. H. Clark* 



Antedon (part) P. H. CARPENTER, Proc. Roy. Soc. London, vol. 28, 1879, p. 384, and following authors. 

 Isometra A. H. CLARK, Proc. Biol. Soc. Washington, vol. 21, 1908, p. 133 (diagnosis; type species 



Antedon lineata P. H. Carpenter, 1888), p. 136 (referred to the Antedonidae) ; Proc. U.S. Nat. 



Mus., vol. 34, 1908, p. 211 (referred to the Antedonidae); Amer. Nat., vol. 42, 1908, No. 500, 



* See also Addenda (p. 836) under 1962. 



