622 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



The distal edge of the radials is visible beyond the rim of the centrodorsal. Each 

 IBi 1 ! is oblong, almost free laterally and strongly incised on the distal border by the 

 IBr 2 (axillary), which is rhombic, broader than long. The division series and proximal 

 brachials have conspicuously flattened lateral faces. 



Syzygies occur between brachials 3+4, 9 + 10, and 14 + 15, and distally at intervals 

 of two muscular articulations. The brachials beyond the fifth have spiny distal edges. 



P! is very short, about 2 mm. long or slightly less. It has 5 or 6 segments, most, 

 if not all, of which are attached to the disk by a web of tissue. On some of the arms 

 P 2 is similar, with 6 segments and measuring 2 mm., but usually it is a genital pinnule 

 and much larger, 5 mm. long. The third and fourth segments of the genital pinnules 

 are more strongly and less symmetrically expanded than those of challengeri, the expan- 

 sion being greater on the distal side. The gonads are fully developed and appear to be 

 testes. 



The arms being broken off rather short, the nature of the ambulacral skeleton is 

 difficult to determine. There appear to be two sets of plates, of which one is of rod- 

 like plates with branched bases and the other of plates which are more fanlike distally. 

 No spicules can be seen in the tentacles. 



Remarks [by A.M.C.]. Since Dr. Dilwyn John (1938) has demonstrated that 

 lineata (i.e., challengeri) is distinct from angustipinna and the differences in the two 

 type specimens are not attributable to the great size discrepancy, I have emended the 

 typescript accordingly. When Mr. Clark concluded that the two were synonymous in 

 1908, no other species, or indeed specimens, of what is now the genus Isometra had been 

 fully described, so that nothing was known of the extent of variation. 



Both of the holotypes of angustipinna and challengeri are mature, though the 

 former has an arm length of only about 25 mm., and both are males. The size and 

 number of segments in the first two pinnules are comparable, allowing for the much 

 greater size of the specimen of challengeri, but the fact that the first genital pinnule is 

 P 2 (or sometimes P 3 ) in angustipinna as opposed to P 6 or P 7 in challengeri is too great a 

 discrepancy to be accounted for in that way, judging from observations on a number of 

 specimens of /. vivipara of varying sizes in which there seems to be no correlation 

 between size and the position of the first genital pinnule. The cirri are very different 

 as a comparison of Carpenter's figures shows. There is also a difference in the texture 

 of the brachials, those of angustipinna beyond the fourth or fifth having the distal edge 

 more or less thorny, whereas in challengeri the arms are smooth. However, this char- 

 acter is probably less reliable. 



Locality. Challenger station 320; off Monte Video, Uruguay (lat. 3717' S., long. 

 53 52' W.); 1097 meters; temperature 2.89 C.; green sand; February 14, 1876 [P. H. 

 Carpenter, 1888; A. H. Clark, 1913; John, 1938] (1, B.M.). 



ISOMETRA HORDEA John* 



Isometra harden. JOHN, Discovery Reports, vol. 18, 1938, p. 132 (in key), p. 188 (stations; description; 

 viviparity) ; text figs. 18, 21, 22; pi. 5, figs. 7, 8. [Not /. hordea FOREST, Beauts du fond des 

 mers, Paris (Larousse), 1955, pi. 50a, which is a Comasterid.] 



Diagnostic jeatures. -The cirri are XL-LXII, 25-75; the first two pinnules are 

 relatively small, especially P!, which does not exceed 5 mm. in length; the first genital 

 pinnule is usually P 6 ; in the female the third to seventh segments of the genital pinnules 



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



