1 S BULLETIN 82, UNITED STATES NATIONAL MUSI. I U 



tn be more or less parallel, then the obliqueness increases again after (he seventh 

 thirteenth brachial, reaching its maximum at from the fifteenth to the twentieth 

 brachials. In the middle portion of the arms t he obliqueness decreases, but it 

 increases slightly again in the distal portion 



THE REVERSION PHENOMENON 



This exterior obliqueness manifests itself by the acute angle formed by the 

 articular line with the longitudinal axis of the arm. facing now outward and now 

 inward. The brachials are therefore longer alternately on the outer and inner sides. 



dly the pinnule is attached to the longer side of the brachial; this is always the 

 case in the middle and distal portions of the anus. In the proximal portion of the 

 arms Gialen pointed out thai the opposite is often found — the pinnules arise from the 

 shorter side of the brachials. Be called this reversion of the articulations. 



This feature is lacking, or is very little developed, in certain Comasteridae, 

 Eudiocrinua, < ialometridae, and Atelecrinus, in which the pinnular side of the brachials 

 i> the longer along the entire arm. Reversion is most conspicuously developed in 

 the Antedonidae and Charitometridae. The second, and usually also the fourth, 

 brachials, bearing I\ and P„ are more strongly developed on the pinnular side, but 

 from the fifth brachial onward reversion appears in a variable number of segments, 

 to be gradually effaced and followed in the middle of the arm by the normal condition. 



Reversion is only slight in Capillastcr sentosa, Comantlius japonica, Tropiometra 

 njra macrodiscus, and Pterometra trichopoda, in which the fifth brachial is only very 

 slightly, sometime-; not perceptibly, narrower on the pinnular side, and the sixth 

 brachial has parallel ends, or an obviously longer pinnular side. The same is the 

 case in Notocrinus tyriliq, in which the sixth or seventh brachial has a longer pinnular 

 aide, and in Clarkometra degans and Stephanomdra spicata in which the seventh or 

 eighth brachial is longer on the pinnular side. The reversion appears more distinctly 

 in Comantheria ddicata grandis, in which the seventh brachial has a longer pinnular 

 side, in St, norm im iliuJ, urn. ( ';/llometra manca, and Himerometra magnipinna, in which 

 the seventh or eighth brachials have longer pinnular sides, in Zygometra elegant, 

 llitirometra crenulnta, and Asterometra anthus, in which the seventh-ninth brachials 

 have longer pinnular sides, and in Liparometra grandis, in which the eleventh brachial 

 first shows a longer pinnular side. 



This phenomenon appears most conspicuously, however, in Pentametrocrinus dio- 

 medeae, in which brachials 4 + 5 have a longer pinnular side, in Isometra vivipara and 

 Hypalonutra ihficta in which the eighth brachial, in Crossometra septentrionalis and 

 Antedon petasus, in which the seventh-ninth brachials, in Diodoniometra bocl:i, in 

 which the tenth twelfth brachials, in Hcliomctra glacialis, in which the twelfth brachial, 

 and in Promachocrinus kergurlrnsis and Monachometra cf. fragilis, in which the thir- 

 teenth brachial fir-t has the pinnular side longer. 



( ii-l'-n said one might BUppose that the Blendemess of the pinnule bases had some- 

 thing to do witli this phenomenon, and this idea seems to be favored by the Calome- 

 tridae, Pedisumetra flavopvrpurea and .Xrometra multicolor having the pinnular side 

 of the brachials longer from the arm base outward. But the lack of reversion in Cmm- 

 tula solans, Comanthus parvicirra, and Aielecrinus cannot, however, be explained in 



