434 BULLETIN 82, UNITED STATES NATIONAL MUSEUM VOLUME 1 



median point on the proximal edge is the opposite apex. On the succeeding brachials 

 the spines become more and more prominent, the spinous area at the same time becom- 

 ing restricted by the moving distally of the proximal apex of the triangle to a point 

 about midway between the proximal and distal borders, and the eversion of the distal 

 edge, now very spinous, becomes more marked. After the proximal third of the arm 

 the spinosity of the dorsal surface of the brachials gradually disappears, leaving the 

 dorsal surface smooth and the brachials with produced and very finely spinous distal 

 edges. 



On the largest specimens there are few or no spines on the dorsal surface of the 

 division series and proximal brachials. 



Syzygies occur between brachials 3+4, 9+10 (from 8+9 to 10+11), 14+15 (or 

 15 + 16), and distally at intervals of from 3 to 11, usually 4 or 5, muscular articulations. 

 The arm width at the first syzygy is respectively 1.3, 1.7, and 2.5 mm. in three spec- 

 imens with arm length about 100, 150, and 200 mm., while the lengths from the 

 proximal edge of the IBr! to the second syzygy are 13.0, 16.5, and 19.5 mm. 



The disk is 15 or 16 mm. in diameter. Ordinarily each groove trunk on 

 leaving the mouth soon divides, and each of these derivatives divides again near the 

 base of the arms so that each of the 5 main grooves about the mouth carries the ma- 

 terial from 4 arms; but there is great variation; the groove formed by the union of the 

 two grooves of an arm pair may run direct to the mouth, or it may join with the grooves 

 from the arm pairs on either side. 



P! is slender and flagellate, from 17 to 26 mm. in length, composed of 40 to 75 

 segments, of which the basal are broader than long, the middle segments are up to 

 twice as long as broad and the distal ones are again shorter. The 6 proximal segments 

 have their corners cut away and the first 15 or so bear numerous ventrolateral spines, 

 which, at least in the smaller specimens, are continued to the tip of the pinnule so 

 that the distal segments appear relatively short. 



P 2 may resemble P! and be of the same length or slightly longer and a little stouter 

 basally, with the same number or rather fewer but slightly longer segments; usually 

 there are 26 to 46, rarely up to 62 segments. P a resembles P 2 , varying with it. 



P 3 is slightly stouter basally than P 2 , with 16 to 40 segments which are propor- 

 tionally much longer than those of P 2 where the latter resembles P,. P 3 may be the 

 first genital pinnule, in which case it is usually much shorter and more tapering than 

 P 2 with all but the basal segments elongated. 



The middle and distal pinnules are 10 to 20 mm. long, not especially slender, 

 with 20 to 26 elongated segments the distal ends of which are sometimes armed with 

 a few spines. 



The gonads are long and slender. 



The ambulacral grooves of the pinnules and arms may be bordered with prominent 

 plates or they may be entirely without calcareous deposits. When the plates are 

 well-developed, there are three or four to each pinnule in a single continuous series, 

 the distal edge of one overlapping the proximal edge of the next. Each is curved in 

 the axis of the pinnule and has its outer part curved over the ambulacral furrow (see 

 part 2, p. 268, fig. 378) . When they are present, the tentacles usually contain numerous 

 knobbed spicules. The plates may be smaller and not in contact. Their occurrence 

 is discussed more fully below. 



