18 BULLETIN 82, UNITED STATES NATIONAL MUSEUM 



to be more or less parallel, then the obliqueness increases again after the seventh- 

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

 brachials. In the middle portion of the arms the 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. 

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

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

 arms Gisl6n pointed out that the opposite is often found the pinnules arise from the 

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



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

 Eudiocrinus, Calometridae, and Atelecrinus, in which the pinnular side of the brachials 

 is 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 P! and P a 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 Capillaster sentosa, Comanthus japonica, Tropiometra 

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

 slightly, sometimes 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 virilis, in which the sixth or seventh brachial has a longer pinnular 

 side, and in Clarkometra elegans and Stephanometra spicata in which the seventh or 

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

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

 side, in Stenometra diadema, Cyllometra manca, and Himerometra magnipinna, in which 

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

 Heterometra crenulata, 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 pmnular side, in Isometra vivipara and 

 Hypalometra defecta in which the eighth brachial, in Crossometra septentrionalis and 

 Antedon petasus, in which the seventh-ninth brachials, in Diodontometra bocki, in 

 which the tenth-twelfth bracbials, in Heliometra glacialis, in which the twelfth brachial, 

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

 teenth brachial first has the pinnular side longer. 



Gisl6n said one might suppose that the slenderness of the pinnule bases had some- 

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

 tridae, Pectinometra flavopurpurea and Neometra multicolor having the pinnular side 

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

 tula Solaris, Comanthus pamicirra, and Atelecrinus cannot, however, be explained in 



