114 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



Comatella, yeocomatcUa, and PalwovomnttUa. 



In the senera Comatella (figs. 15'2. 154. 156. 1T4, pp. S3. 89), Neocomntella, and 

 PalcEocomateUa a curious condition occurs. Typically only the outer derivative 

 from each axillary bears a division series, the inner remaining undivided. Care- 

 ful examination shows that on these inner undivided branches the first two 

 brachials are always united by syzygy, while the following brachials are united 

 by muscular articlation. Thus following the last axillary we find on the inner 

 side an arm which begins with a syzygial pair and on the outer an arm beginning 

 with a pair of ossicles united by synarthry (often replaced by a syzygy) followed 

 by a pair united by syzygy. 



The only possible interpretation of this condition is that, considering only 

 the outer derivatives from each axillary, there occurs the structure normal for 

 species in which the division series are all of two elements; but considering the 

 inner derivatives from the same axillaries, the division series themselves act 

 also as the first brachial pair of each of the arms, which therefore appear to begin 

 with a syzygial pair. 



Thus while in CapiUaster and Nemaster nonnally the first element of the 

 pair of ossicles immediately succeeding the IIBr and all following axillaries is 

 suppressed, and sometimes both elements are wanting, in Comatella, Neocomatella, 

 and Palceocomatella normally both the elements of the pair of ossicles immedi- 

 ately succeeding the IIBr and all following axillaries are suppressed on the inner 

 derivative, though both are present on the outer. 



In the species of Comntvla with more than 10 arms (figs. 121, p. 79, and 

 1020, pi. 8) the arm division appears to be similar to that of Comatellu. 



There is in the British Museum a curious example of CapiUaster viultiradiata, 

 unfortunately without data, in -which three of the IIBr series are 2, the other 

 two being 4(3+4) : each of the latter bears a IIIBr 3(2-i-3) series; the IIBr 2 

 series may be immediately followed by a syzygial pair (the normal third and fourth 

 brachials), or they may give rise to undivided arms in which the first syzj'gy is 

 between the second and third brachials. 



Thus this specimen presents the characteristic features of the arm division 

 of Comatella combined with those of the arm division of CapiUaster. 



Distal arm division. 



In Metacrmus (figs. 161, 162, p. 83) the first brachial pair as defined in the 

 preceding pages immediately follows the radials as in the Pentametrocrinidse 

 and in Atopocrinus, but beyond this, at increasingly long intervals, the arms 

 divide many times. 



This type of arm division does not occur regularly in any of the recent 

 comatulids, but is found as a not uncommon sporadic variant in many of the 

 Oligophreata. and rarely in the Macrophreata, instances being recorded in Tropio- 

 metra picta (fig. 173, p. 89), where it is not unusual, Dichrometra doderleini (fig. 

 1027, pi. 11), Gatoptometra ophiura, Antedon hifida, and Thaumatocrmus, sp., 

 in the last occurring twice in a single arm. 



