82 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Distichal radii. A term sometimes employed to include a single IIBr series and all 

 the derivatives from it; it is therefore equivalent to one-half of a "ray," the 

 latter term covering all the derivatives from a single radial (sec fig. 29, p. 71). 



Distichals (IIBr). The secundibrachs ; the post-costal segments as far as, and 

 including, the next following axillary (see fig. 29, p. 71). 



In the comatuhds the distichals arc usually two in number, the two being 

 united by synarthry; this is expressed "IIBr 2;" more rarely these two are 

 united by syzygy, the expression in this case being "IIBr 2(1 + 2);" but they 

 may be doubled, in which case the second pair are united by syzygy, the for- 

 mula being "IIBr 4(3 + 4)." 



When distichals are present the most distal is always an axillary, except in 

 Uintacrinus. 



Distichium. Same as a Distichal radius. 



Division series. (1) A term used to designate all the ossicles collectively between 

 the radials and the first segments of the free undivided arms (see fig. 29, p. 71). 



(2) A term occasionally used to designate all the elements collectively 

 between the first or IBr ("radial" or "costal") axillary and the first segments 

 of the free undivided arms. 



(3) A term sometimes employed to designate any one of the series of 

 ossicles which terminate in an axillary. 



Dorsal. Same as Aboral. 



Dorsal cirrlii. See Cirri. 



Dorsal cirri. See Cirri. 



Dorsal interradial furrows. The shallow grooves or furrows on the dorsal surface 



of the radial pentagon which lie over the sutures between the radials and accom- 

 modate the basal rays (see figs. 483, p. 365, and 512, p. 373, and pp. 370-372). 

 Dorsal ligament fossa. The large semicircular fossa or depression occurring in a 



muscular articulation dorsal to the transverse ridge (see figs. 9-11, p. 65, 31-32, 



p. 71, and 431, 432, p. 349, and pp. 114, 376). 

 Dorsal nervous system. The nervous system lying entirely within the primary 



skeletal elements. 



This nervous system corresponds to the subcesophageal ganglion and the 



ventral nervous system of the annelids, crustaceans, insects, etc. 

 Dorsal pole. The center of the dorsal surface of the centrodorsal ; that part of the 



centrodorsal wlu'ch is bare of cirri. 



It is usually smooth, and may be flat, concave, or convex (see figs. 9, p. 



65, 146-150, p. 220, 151-159, p. 221, 171, 173, p. 231, 183, 185, 187, 189, p. 



235, 191, 193, 195, 197, p. 237, and 199, 201, 206, p. 239, and pp. 230-232). 

 Dorsal radial furrows. The furrows on the dorsal surface of the radial pentagon 



wlu'ch traverse the center of the radials along their longitudinal axes (see figs. 



434, 4456, p. 351). 

 Dorsal spines. Spinelike projections from the dorsal (lower) surface of the cirrus 



segments; they are not always present, and if present are usually found only 



on the outer cirrus segments (see figs. 4, p. 63, 333, p. 283, and 347-348, p. 



289, and pp. 276-284. 



