PART 5 A MONOGRAPH OF THE EXISTING CRLNOIDS 319 



This character is by no means invariable, for the third syzygy on individual arms 

 may fall anywhere from between brachials 12+13 to between brachials 16 + 17; but it 

 is found to hold true in the great majority of cases. 



Commonly associated with this species is the much smaller Retiometra alascana 

 which bears a superficial resemblance to its young. R. alascana may be distinguished 

 at once, however, by the very slender and greatly elongated PI which is twice as long as 

 Pz and is composed of greatly elongated segments with spinous distal ends, by the occur- 

 rence of a large gonad on P 2 , which is of the same length as and similar to the succeeding 

 pinnules, and by the distal intersyzygial interval of only 2 muscular articulations. 



Description. The centrodorsal is flattened hemispherical, 8 mm. in diameter at the 

 base, witli a deep pit at the dorsal pole 2 mm. in diameter. The sides of the centro- 

 dorsal, up to the borders of this pit, are entirely covered with cirrus sockets which de- 

 crease slowly in size distally. 



The cirri are L-LXXX, 42-53, from 45 to 70 mm. in length, moderately stout. 

 The first segment is about two and a half times as broad as long, the second is from 

 half again to twice as broad as long, the third is slightly broader than long, and the 

 fourth is as long as broad or somewhat longer than broad, while the fifth is slightly 

 longer; the sixth and following are about twice as long as broad or slightly longer, the 

 segments in the distal fourth of the cirri slowly decreasing in length so that the last two 

 are about as long as broad. The opposing spine is represented by a small terminal 

 tubercle. The terminal claw is longer than the penultimate segment, moderately stout 

 and only moderately curved; it decreases rapidly in diameter in the proximal half, but 

 much less rapidly in the distal so that the profile of the ventral surface is regularly con- 

 vex, while that of the dorsal is a rounded angle. In the proximal half the cirrus seg- 

 ments have rather prominent ends and the ventral profile is slightly concave. In the 

 distal half these features disappear, the segments acquire a polished surface, become 

 gradually more compressed laterally, and the middle of the dorsal portion of the distal 

 edge becomes prominent. On the last twelve segments the dorsal rnidline is roundedly 

 carinate, ending distally in a prominent tubercle. The last three segments decrease 

 slightly in size. 



The distal edges of the radials are even with the border of the centrodorsal. 



The IB^ are very short, not quite in contact laterally, five or six times as broad as 

 the median length, the sides strongly converging distally and making an angle of about 

 90 with each other. The IBr 2 (axillaries) are about as broad as long, as wide as the 

 base of the IBr! so that the lateral angles extend far beyond the anterolateral angles of 

 the IBr,, with a produced but truncated distal angle and a rounded posterior process 

 which rises to a fairly prominent synarthrial tubercle on the border line between the 

 IBr i and the IBr 2 . The distal edge of the IBr! and the proximal and distal edges of the 

 axillaries are rarely smooth ; they are usually finely spinous, sometimes coarsely denticu- 

 late or spinous. 



The 10 arms reach a length of 235 mm., and are composed of up to 330 brachials. 

 The first brachials are almost triangular in shape, their sh.ort inner sides, which just 

 meet over the anterior angle of the axillary, lying in the same straight line. The second 

 brachials are triangular, with the distal border slightly longer than the other two, the 

 interior angles almost meeting and forming a conspicuous opening bounded by the inner 

 edges of the first brachials, lying in a straight line, and the inner edges of the second 

 extending inward and upward from them at an acute angle. The first syzygial pairs 



