PART 5 A MONOGRAPH OF THE EXISTING CRINOIDS 197 



away distally. The profile of the dorsal edges of the segments is slightly concave or 

 almost straight. (Some details of proportions of the cirri are given in table 6 (pp. 229- 

 230) for comparison with A. bifida moroccana.) 



Radials even with the edge of the centrodorsal in the median line, but visible in the 

 angles of the calyx. 



A compact group of 3 to 7 or more (usually 3 or 4) small plates is frequently present 

 in the interradial perisome between the ossicles of the IBr series. W. B. Carpenter 

 states that there is no trace of these plates in specimens from the Clyde, Strangford 

 Lough, or Kirkwall Bay, but he found them in specimens from Ilfracombe and from 

 Plymouth Sound; they are described and figured by J. S. Miller (1821) in specimens 

 from Milford Haven, and in a specimen from Shetland dredged by Mr. Barlee and sent 

 to Sir Wyville Thomson (W. B. Carpenter, 1866) they were present in three of the 

 angles and absent from the other two. Sir Wyville Thomson (quoted by Norman, 

 1865) says that mature individuals usually have no trace of these plates, but frequently 

 there are groups usually of three in the spaces between the IBr axillaries. They are 

 given as present without comment in the specimens from Arran and the mouth of the 

 Mersey listed under A. milleri. Disk naked, or with scattered tubercles containing 

 groups of radiating spicules. 



IBr! very short, bandlike, usually over four times as broad as long, decreasing 

 rapidly in diameter distally, proximally just in contact at their bases, the sides of ad- 

 jacent ones rapidly diverging; from this point. IBr s triangular, rather broader than 

 long, the anterior angle somewhat produced, and with a rounded posterior process 

 incising the IBri with which it rises into a rounded moderately prominent tubercle; 

 proximally the IBr 2 are as broad as the bases of the IBr, so that their posterior edges 

 overhang on each side the narrowed distal borders of the IBr, (see fig. 13,g,/i). 



The ten arms, which are composed of about 140 brachials, are usually between 

 50 and 65 mm. in length; the first brachial is wedge-shaped, much longer outwardly 

 than inwardly, incised in the median line by a posterior projection from the second; 

 interiorly the bases of adjacent first brachials meet above the apex of the IBr axillary, 

 their inner sides thence diverging at right angles to the line of contact between them; 

 exteriorly the first brachials are produced into a narrow ventrolateral ridge; the second 

 brachials are irregularly quadrate; the third and fourth form a syzygial pah- which 

 is somewhat longer inwardly than outwardly and about twice as broad as its outer 

 side; the fifth to the eighth brachials are wedge-shaped, though the ends are not es- 

 pecially oblique, about twice as broad as their median length; the ninth and tenth, 

 forming the second syzygial pair, are more obliquely wedge-shaped; the following are 

 triangular, not quite so long as broad, after the proximal third of the arm becoming 

 wedge-shaped, slightly broader than long, the proportion of length to breadth gradually 

 increasing distally while at the same time the ends of the brachials become less and less 

 oblique so that in the distal portion of the arm they are about twice as long as broad 

 with the ends only slightly oblique and with the articulations somewhat swollen. At 

 about the second syzygy the distal edges of the brachials develop a finely dentate border 

 which begins gradually to disappear after the middle of the arm. The synarthry be- 

 tween the first and second brachials rises to a broad tubercle resembling the one on the 

 line between the two elements of the IBr series, and the articulations between the fol- 

 lowing three or four brachials are usually somewhat swollen over the ends of the apposed 

 fulcral ridges. 



