UIXTACRINUS: ITS STRUCTURE A.ND RELATIONS. 19 



Arm. Specimen No. 178. G6 mm. 



I. 9 13 17 21 25 29 33 38 44 49 54 

 II. 9 12 15 18 21 25 30 36 42 48 54 59 66 72 



III. 10 14 16 22 27 34 39 45 



IV. 9 12 16 21 26 31 37 42 49 56 62 65 69 74 78 

 V. 10 12 15 19 24 29 34 40 47 .54 61 66 71 75 



Single arm of Specimen I xx. 65 mm. 



10 14 18 22 27 33 39 43 48 52 56 61 66 71 77 82 88 97 110 

 115 127 134 140 145 153 159 168 177 184 191 199 204 210 215 224 230 238 244 

 252 258 (about 50 vacant) 307 315 323 331 339 346 355 360 367 373 379 385 391 

 398 407 415 421 427 434 440 446 452 458 466 



The distribution of syzygies can be studied on the photograph (PI. VIII.) 

 almost as well as upon the specimens themselves. The arms of several 

 specimens are shown in the proximal portions, and the isolated parts of 

 arms at A 3 and A 5 belong to specimen I xx, and end at a point about 24 

 inches from the calyx. 



The syzygies are also well shown in specimens 117, 3, 78 rt, and XX « 

 (PI. v., Figs. 1, 2, 3, 4). In No. 78 a, which is a young individual, it will be 

 observed that the syzygies commence lower than in the others, — the first 

 syzygial pair being the 6-7th. 



The "skewino;" of the brachials, resultin"' f rom the more or less diagonal 

 position of the fulcral ridge upon the articulating faces of the plates, wliich 

 has been well described and illustrated by Mr. Bather, is thoroughly shown 

 by my specimens. The effect of this structure is to give extraordinary 

 mobility and flexibility to the arms (PI. VII., Fig. 8). 



In the number of arms there is almost no variation. In one case, speci- 

 men Xe, the first fixed pinnule has developed into an extra arm — thus 

 making eleven arms in all (PI. VI., Fig. 9); in another there are two extra 

 arms similarly developed; in another — XIV c — there is an arm wanting 

 (PI. VI., Fig. 8) ; but these are the only departures from the normal number 

 observed. 



As to other variations from normal, I may mention that in one specimen, 

 No. 1, the ill Br plates are wanting in one ray, and in place of them is a 

 large pinnule springing out of the axil (PI. VI., Fig. 7); in another, ly, the 

 illBr plates are wanting, and the space is unoccupied ; in another, I ii, two 

 of the iBr spaces are anchylosed into a solid, shield-like plate, the suture 

 lines between the plates being faintly visible as raised lines. 



The ventral side of the arms and pinnules has a broad, rather shallow 



