ECHINODERMS OF SMITHSONIAN-HARTFORD EXPEDITION — CLARK 451 



bricating disk scales are of moderate size, but the radial shields 

 are very small, oblong with rounded ends, two to three times as long 

 as broad, and widely separated. They are situated on either side 

 of the arm bases, and the two of each pair diverge inwardly. The 

 lateral border of the disk between the arm bases is sharply keeled. 

 The first tentacle pore carries 1, rarely 2, tentacle scales; those fol- 

 lowing have one only. The first side arm plate carries 2 arm spines, 

 the second carries 3, and the third usually has 4; there are 3 arm) 

 spines on the side arm plates succeeding as far as the middle of the 

 arm, beyond which the number drops to 2, with an occasional 3. In 

 the proximal portion of the arm the uppermost arm spine is equal 

 in length to about two arm joints, the lowest to one, with the middle 

 arm spine intermediate. The arm spines are very slender. As the 

 arm joints become longer distally the arm spines decrease in rela- 

 tive length, the upper soon equaling one and one-half arm joints with 

 the lower somewhat shorter. Later the upper is only slightly longer 

 than an arm joint. In the distal portion of the arm the two arm 

 spines are of the same length, about as long as an arm joint. Ter- 

 minally the upper arm spine decreases in length, finally becoming 

 only about two-thirds the length of the lower. In color the disk 

 is dark brown, the arms above light brown, the circumoral struc- 

 tures, spines, and lower surface of the arms white or slightly yellow- 

 ish white. The naked disk and long and very slender arm spines 

 easily distinguish the young of this species from the young of 0. 

 echinata and of O. pumila. 



OPHIOCOMA PUMILA Lutken 



Plate 54, Figuee 3 



Localities. — Smithsonian-Hartford station 12, Haiti (33, E. 5526 

 to E. 5558). Station 16, Puerto Rico (1, E. 5560). Station 23, St. 

 Thomas (2, E. 5563, E. 5564). Station 28, St. John (1, E. 5559). 

 Station 56, Barbados (2, E. 5561, E. 5562). 



Notes. — The specimen from station 28, St. John (E. 5559) (pi. 54, 

 fig. 3), has the disk 4 mm in diameter and the arms about 13 mm long. 

 There are 6 arms, the 3 on one side slightly larger than the 3 on the 

 other side. The granules aborally are high, twice as high as thick 

 or higher, swollen-conical, and somewhat less densely placed than in 

 larger examples. There are 4 arm spines until near the end of the 

 arms, where the number falls to 3, Except for the possession of 6 

 arms, the young of this species more closely resemble the fully grown 

 than do the young of 0. echiTiata or 0. riisei. The occurrence in this 

 individual of two groups of 3 arms of different sizes suggests that 

 autotomy may take place at least twice before the ultimate 5-armed 

 stage is reached. 



