460 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



The second brachial is larger, sometimes considerably larger, with the middle of 

 the proximal border depressed into an obtuse angle entering the concavity on the 

 distal border of the first, so that it is irregularly quadrate in dorsal view. 



The first syzygial pair is usually longer interiorly than exteriorly; in extreme 

 cases the hypozygal (third brachial) may be reduced to a low triangle with the apex 

 just reaching the outer edge of the arm. In length it varies from as long as broad in 

 the smaller species to twice as long as broad in the largest. 



The next 2 brachials are oblong or slightly wedge-shaped, from about as broad as 

 long to twice as broad as long, and those following rapidly become very obliquely 

 wedge-shaped or triangular and as long as, or somewhat longer than, broad. Dis- 

 tally the ends of the brachials become gradually less and less oblique and the relative 

 length increases so that in the terminal portion of the arms the brachials are elongate 

 with almost or quite parallel ends and more or less strongly swollen articulations. 



The distal ends of the brachials are sometimes, though rarely, smooth; usually 

 they are more or less conspicuously bordered with fine spines, and the central and outer 

 portion of the distal brachials may be covered with fine longitudinal striations. 



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

 of from 2 to 6 (usually 2 or 4) muscular articulations. The position of the second and 

 third syzygies and the distal intersyzygial interval in the larger species is somewhat 

 less definitely fixed than in the other subfamilies of Antedonidae. 



One species (Perometra a/ra) has only been found with 13 or more arms, the IIBr 

 series being 2; but all the other species in the subfamily are 10-armed. 



The oral pinnules in this subfamily show very considerable uniformity. The most 

 interesting feature is the uniform absence of P a in one species of Erythrometra and in 

 Hypalometra, and the usual absence of this pinnule in another species of Erythrometra 

 and in one of the three species of Perometra. In Hypalometra, Pj, as well as P a , is 

 invariably absent. The absence of these pinnules is undoubtedly correlated with the 

 unusually small angle between the planes of the division series and arm bases and the 

 dorsoventral axis characteristic of this group. 



PI is the longest and stoutest of the oral pinnules. In nearly all the species P! is 

 considerably more robust than is usually the case in the Antedonidae, somewhat 

 stiffened, and composed of 12 segments, all but the first of which are twice as long as 

 broad and have more or less spinous distal ends. In one species of Nanometra Pj has 

 15 to 20 segments which are mostly two and a half times as long as broad and is quite 

 smooth, resembling PI in certain species of Psathyrometra; but in the other species of 

 the same genus P l has only 8 to 12 segments and resembles the same pinnule in the 

 other genera. 



P 2 is usually shorter than Pjj it may be only slightly shorter, but sometimes is 

 only half as long. At the same time it is proportionately less stout than P, with some- 

 what fewer segments, from nearly as many to about two-thirds as many, which have 

 more spinous distal ends. In Perometra afra and P. diomedeae only is P 2 similar to P t . 



P 3 is similar to P 2 but more or less shorter; the component segments have more 

 spinous distal ends. 



The distal pinnules are from half again as long to only slightly longer than (rarely 

 about as long as) P : , and are commonly composed of twice as many segments, though 

 sometimes the number is only slightly larger. Generally the distal pinnules are rather 

 stouter than is usual in the Antedonidae, and are composed of less elongated segments. 



