240 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Key to the Brittle-Stars of Porto Rico. 
A. Arms not capable of being vertically coiled; disk sharply set off from arms. 
I. Arm-spines short, smooth, on distal edge of side arm-plates, closely appressed to arm, 
a. Disk covered with fine granules. 
ol. Arm-spines 8 to 10, short, broad, equal; arms comparatively short, about 3 times diameter of disk; 
radial shields covered; adoral plates covered by granules. . Ophiura brevicauda. (2) 
a2. Arm-spines 7 to 8, slender, equal; arms more than 3 times the diameter of disk; radial shields covered; 
adoral plates naked Ophiura brevispina. (3) 
a3. Arm-spines 8 to 9, the lowest the longest; radial shields covered Ophiura appressa. (1) 
ct4. Arm-spines 6 to 8, the lowest the longest; radial shields naked; color ashy gray Ophiura cinerea. (4) 
a5. Arm-spines 8 to 9, the lowest the longest; radial shields naked: color purplish red Ophiura rubicunda. (5) 
b. Disk covered with scales or plates. 
61. Upper arm-plates with supplementary plates on each side; arm spines 4 to 6 Ophiolepis elegans. (6) 
62. Upper arm-plates, without supplementary plates. 
c. Radial shields separated from genital plates by a pair of plates, which bear on their proximal edge 
a row of bead-like papillae, similar to those on distal edge of radial 
shield. This makes the radial shields appear as though cracked trans- 
versely, each side of the crack bearing a row of little papillae. 
Ophiothyreus goesii. (10) 
cc. Radial shields not as above. 
d. No tentacle-pores beyond the basal arm joints. 
e. Disk-scales rough and swollen; an erect spine on each of several basal arm-plates; arm-spines 2. 
Ophiomusium sculptum. (14) 
ee. Disk-scales smooth; under arm plates persisting nearly to end of arm; arm-spines 3. 
Ophiomusium eburneum elegans. (13) 
eee. Disk-scales smooth, rather few; no under-arm plates beyond third or fourth; arm-spines 4 to 5. 
Ophiomusium validum. (15) 
(id. Tentacle pores to end of arm. 
/. Not more than 2 tentacle-scales. 
g. Small separated radial shields; 4 to 5 arm-spines, lowest longest Opliiozona impressa. (7) 
gg. Radial shields large, slightly separated; arm-spines 3 (rarely 4) subequal ... Opliiozona nivea compta. (8) 
ggg. Radial shields large, with a blunt spine on the outer end; arm-spines 3 Opliiozona tessellata. (9) 
ff. Tentacle-scales 3 or more. 
h. Papillse of arm-comb small, almost bead-like i . . . Ophioglypha robusta. (12) 
hh. Papillse of arm-comb sharp and cylindrical Ophioglypha ljungmani. (11) 
II. Arm-spines rather long, more or less at an angle to arm, and often rough or thorny. 
a. Disk covered with little granules. 
ol. Disk granules coarse and uneven; no tooth papillse; basal oral papillse very wide; arm-spines 3. 
Ophiostigma isacanthum. (40) 
a2. Disk granules even; numerous tooth-papillse; arm-spines 4 to 6. 
b. Two tentacle-scales Ophiocoma cchinata. (19) 
bb. One tentacle-scale. 
c. Color almost black; under side and especially tentacles rusty red Ophiocoma piisei. (21) 
cc. Color light brown; arms banded with darker Ophiocoma pumila. (20) 
aa. Disk more or less covered with scattered spines, spinules or thorny stumps. 
61. No oral papillse; arm-spines 5 to 10, long, glassy, thorny. 
c. Disk beset with numerous trifid stumps, among which there are often a few slender spines. 
Ophiothrix angulata. (16) 
cc. Disk beset with slender spines Ophiothrix mrstedii. (17) 
ccc. Disk with a few long slender spines; arm-spines very long, slender, rough and glassy. . Ophiothrix suensonii. (18) 
6 2. Oral papillse few, rarely more than 2 on a side; arm-spines 4 to 7, smooth to the naked eye. 
cl. Size large, disk over 10 mm.; color very dark; arms 5, very long; arm-spines 7 Ophiactis longibrachia. (25) 
del. Size small, disk not often over 7 mm.; colors green and white; arms usually 6, rather 
short; arm-spines 7 Ophiactis krebsii. (24) 
cldd. Size small; colors purple and white; 6 arms; arm-spines 4 Ophiactis loricata. (26) 
6 3 Oral papillae numerous, 4 or more on each side. 
e. Oral shield touches the side arm-plate on each side. 
/. Radial shields long, narrow, and partly covered. 
g. Arms, 5. Rows of spines closely approximate dorsally Ophiacantha bidentala. (41) 
gg. Arms, 6. Rows of spines not approximate dorsally Ophiacantha ophiactoides. (42) 
ff. Radial shields broad and thin, one overlapping the other Ophioplinthaca spinissima. (45) 
ee. Oral shield separated from the side arm-plate by a corner of the adoral-plate Ophiopristis hirsuta. (44) 
e. Disk covered With scales or smooth skin, without spines or thorny stumps. 
« 1. Upper arm-plates with supplementary plates on each side. 
li. Color pale bluish, yellowish, or whitish; arms yellowish, banded with brown-; 
usually a network of fine brown lines on disk Ophionereis reticulata. (39) 
li.h. Color reddish-brown; arms banded with darker Ophionereis dubia. (37) 
hhli. Color olive-green, spotted with yellowish Ophionereis olivacea. (38) 
