A MONOGRAPH OF THE EXISTING CRINOIDS 173 



The two small species parva and minima have 10-15 cirrus segments, P 2 with 8-11 

 segments, and the arms 35^15 mm. long. It is possible that they represent the same 

 species. 



Decametra brevicirra is intermediate between the group including mylitta and 

 mollis and that including parva and minima, having 21-23 cirrus segments, but only 

 12-13 segments in P 2 . The arms, however, are only 35 mm. long, indicating that the 

 species is based upon an immature individual. Just what its status is must be left for 

 future determination. 



The form herein described as Oligometra serripinna var. macrobrachius I originally 

 regarded as a species of Decametra. As it is sufficiently like some species of Decam- 

 etra especially mylitta and mollis to be easily confused with them I have, as a 

 precautionary measure, inserted it in the key to the species of Decametra. 



KEY TO THE SPECIES IN THE GENUS DECAMETRA 



a 1 . Longest cirri with more than 15 segments. 

 6'. More than 25 cirrus segments. 

 c 1 . Pj much longer than, often nearly or quite twice as long as, PI; basal segments of P 2 not 



carinate; arms 80-110 mm. long. 



d l . Cirrus segments subequal, from half again to twice as broad as long, or sometimes the dis- 

 tal only slightly broader than long; cirri XV-XX, 26-28, 12-14 mm. long; P, 5-8 mm. 

 long with 13-21 segments; "Pi much larger, stouter, and stiffer, 9-12 mm. long with 

 16-18 segments; Pj similar to P 2 but smaller; arms 90-100 mm. long (Cargados Carajos; 



55 meters) alaudae (p. 183) 



d 2 . Distalmost cirrus segments about as long as broad. 



e 1 . Most of the segments of P 2 are from one-third to one-half again as long as broad with 

 prominent, though not spinous, distal ends; cirri XIV-XXI, 23-29, 12-13 mm. long; 

 PI 4.5 mm. long with 14 segments; P 2 8 mm. long, stouter and stiffer than P! though 

 not especially enlarged, wtih 15-17 segments; Pa 6 mm. long with 14 segments; arms 

 about 80mm. long (Ceylon and the Maldive Islands; 0-58 meters)_taprobanes (p. 185) 

 e 2 . Most of the segments of P 2 are twice as long as broad, with spinous distal angles; cirri 

 XIX, 26-27, 13 mm. long; PI very small and weak, 4.5 mm. long; P 2 9 mm. long with 

 15 segments; P 3 similar to P 2 , 6-7 mm. long (Muscat, Arabia; littoral) arabica (p. 187) 

 c 2 . PI nearly as long as P 2 ; Pa similar to P 2 but a little smaller; aboral margins of the basal seg- 

 ments of P 2 more or less produced as flattened spinulose projections; arms about 70 mm. 



long (off Clarence River, New South Wales; 64-66 meters) zebra (p. 175) 



6 2 . Cirri with 16-26 (usually between 18 and 24) segments. 



c 1 . More or fewer of the distal cirrus segments with paired tubercles dorsally; P 3 (except in 



brevicirra) longer than PI. 



d 1 . Third to seventh or eighth and following cirrus segments about as long as broad, the 

 terminal segments sometimes slightly longer than broad; segments of P 2 with conspicu- 

 ously everted and spinous distal ends; lower pinnules to at least Pj stiffened. 

 e 1 . Segments of P a beyond the basal not greatly elongated, not becoming more than twice 

 as long as broad; pairs of tubercles on the distal cirrus segments often indistinct or 

 more or lesss fused; cirri XII-XXX, 18-26 (usually 20), 10-13.5 mm. long; P, 4-5.8 

 mm. long with 9-17 segments; P 2 6.8-10 mm. long with 12-20 segments; P 3 about as 

 long as PI with 13 or 14 segments; arms 40-80 mm. long (southern Japan from Kago- 



shima to Sagami Bays; 0-137 meters) tigrina (p. 178) 



e 2 . Segments of P 2 much elongated, from the fourth onward being between 2 and 5 times 

 as long as broad; paired tubercles on the distal cirrus segments very distinct; cirri 

 XVI, 22-24, 12-14 mm. long; P, 7.5-8 mm. long with 16 segments; P 2 13 mm. long 

 with 20 or 21 segments; Pa 7.5 mm. long with 15 segments; arms about 90 mm. long 



(Suez Bay; 18 meters) chadwicki (p. 176) 



d 2 . Cirrus segments all broader than long, or the terminal sometimes becoming nearly or 



