110 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



as Primibrachs (IBr), Secundibrachs (IIBr), Tertibrachs (IIIBr), Tetrabrachs (IVBr), 

 etc. The first series (absent in the family Pentametrocrinidse and in the genus 

 Atopocrinus of the Atelecrinidfe) is invariably composed of two elements, and it is 

 therefore an easy matter to detect the concealment of the radials by counting 

 backward from the first post-radial axillary, except in the five-armed genus Eudio- 

 ainus in which the first division series, though present, does not terminate in an 

 axillary (see figs. S3, p. 136, 84, p. 137). 



(6) The Free undivided arms (see fig. 29, p. 71) arise from the final axillaries, 

 and are composed of a linear series of (as viewed dorsally) wedge-shaped and 

 triangular, or more or less oblong, brachials which end in a growing tip. 



In certam species of the family Comastcridaa from one to six of the arms may 

 end m an axillary bearing a pair of piimules (see figs. 456, p. 79, 47, p. 81); such 

 anns may be recognized by the entire absence of ambulacral grooves, and by their 

 shortness, they sometimes being not more than one-tliird, and often not more than 

 one-half, as long as normal arms (see fig. 45, p. 79). 



(7) The Disk (see figs. 15-19, p. 67, and 117, p. 183) is the adoral (ventral) 

 covering of the internal organs, and appears to unite the bases of the arms on 

 their ventral side; it is exactly opposite in position to the centrodorsal. The 

 perisome of the disk is continued down between the division series to the radials, 

 and outward along the ventral surface of the arm to the tip, as well as along the 

 ventral surface of the pinnules almost to their tips. 



The disk is sometimes pentagonal or more or less cii-cular m outline (see figs. 



15, 19, p. 67), the outer borders of the interambulacral areas being straight or 

 slightly convex; but often the outer borders of the interambulacral areas are 

 strongly concave so that the disk becomes approximately stellate m shape (see figs. 



16, 17, p. 67); in the latter case the disk is said to be incised. 



The ventral perisome of the outer, and usually the middle, pinnules, and of 

 the arms is almost invariably marked in the median line by a deep furrow, the 

 Ambulacral groove (see figs. 15-19, p. 67, and 45a p. 79) ; the grooves from the various 

 arms of each ray converge and unite upon the disk, fonnmg five radiating grooves, 

 wluch themselves converge to the central or subcentral Mouth (see figs. 15-19, p. 67) ; 

 the latter may be readily distinguished as a round, oval, or crescentic opening in 

 the center of the converging ambulacral grooves. 



In the Comasteridte and Umtacrinidre the ambulacral grooves from the arms 

 usually lead into a horseshoe-shaped or crescentic furrow about the margin of the 

 disk, the mouth being at or near the center of this furrow and therefore marginal 

 (see figs. 25-28, p. 69), and many of the species belonguig to the first-named 

 family are further peculiar in that ambulacral grooves are often entirely absent 

 from the posterior rays, and sometimes from many or all of the anns arising from 

 the otiier raj-s (see figs. 27, 28, p. 69, and 45, p. 79). 



When the surface of the disk is divided by five subequal converging ambulacral 

 grooves into five roughly triangular Interambulacral or Interpalmar areas (see figs. 

 15-19, p. 67), one of these is usually seen to be sliglitly larger than the rest and 

 to contain, at or near the center of its margin a conical prominence, perforated at 

 the tip, the Anal tube (see figs. 15-19, p. 67); this area, which includes the anal 



