312 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



ing are almost triangular and usually half again as broad as long, and the distal are 

 obliquely wedge-shaped. 



Syzygies occur between brachials 1 +2, on the 2 exterior arms of each postradial 

 series also between brachials 3 + 4, again between brachials 9 + 10 or 11 + 12, the latter 

 position being common on the outer arms, and distally at intervals of from 2 to 7 

 (usually 4 or 5) muscular articulations. 



Pi is long and tapering, up to 20 mm. in length, and consists of about 60 short 

 segments, of which the lowest are rather broad and stout with prominent dorsal 

 edges. It bears a large terminal comb. The next 4 or 5 brachials bear similar pin- 

 nules, which decrease rather rapidly in size and bear progressively smaller combs. 

 Two, or sometimes three, of the basal segments are rather strongly carinate. The 

 succeeding pinnules are less slender, with stouter segments, and increase slightly in 

 length. 



The mouth is radial and almost marginal. The anal area is more or less thickly 

 covered with irregular plates, but the ambulacral grooves are unprotected. 



The color in alcohol is reddish, yellowish, or grajdsh brown, bleaching to white, 

 often with a dark mediodorsal stripe. 



Description of young specimens. — In this species the 10 additional arms do not 

 arise by adolescent autotomy as in the other multibrachiate comatulids, but in the 

 small 10-armed young the second brachial develops, instead of the usual pinnule, an 

 additional arm which gradually increases in size until it reaches the same dimensions 

 as the original arm. The equalization of the arms, however, does not occur until the 

 animal has reached almost the full size. 



This manner of increasing the number of arms is, so far as known, unique among 

 the recent comatulids, but there is a close parallel in the case of Promachocrinus ker- 

 guelensis (vol. 1, pt. 2, pp. 530-557), in which the so-called "interradial" rays and 

 arms do not begin to form until after the first 5 (radial) rays and the 10 arms which 

 they bear have attained a very considerable size, and do not attain the length and 

 stoutness of the latter until the animal is almost fully grown. 



Carpenter remarked that the 2 outside arms of each postradial series in young 

 individuals are often much smaller than the 2 inner arms, and noted that this is espe- 

 cially distinct in those from the Aru Islands, in one of which, with an arm length of 

 100 mm., the outside arms are so small as to look like unusually developed pinnules. 



In the youngest of the Challenge?- specimens from the Aru Islands as described 

 by Carpenter there is a relatively large pinnule on the second brachial and a very 

 small one on the third, but there are none on the next 4 brachials, though they 

 reappear again on the eighth. 



In 2 young specimens from Holothuria Bank and 1 from Baudin Island there 

 are no cirri, but the centrodorsal is somewhat raised above the dorsal surface of the 

 radial pentagon. Another specimen from Holothuria Bank possesses a few small, 

 slender and weak cirri, each with 12 segments. 



On each of the 10 arms Pj is replaced by a small arm which is generally about 

 two-thirds the length of the original arm. 



In the specimen which served as the type of Comatula etheridgei the small outer 

 arms are usually about two-thirds the length of the original arms, but they may be 

 three-quarters of the length of those arms. 



