64 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



this ambulacrum usually, at the base of the arm, makes a more or less abrupt 

 turn to the right to reach the interradial mouth, which is situated between 

 the bases of the anterior and the right anterior arms (see figs. 22-27, p. 69, 

 and 117, p. 183, and p. 110 [6]); (see Axis and Orientation). 



Anterior radii. (1) The radius in which the anterior arm is situated is commonly 

 distinguished as the anterior radius (see fig. 22, p. 69). 



(2) It is sometimes convenient to differentiate the radii on either side of 

 the anal area from the three others, in which case there are distinguished 2 

 posterior and 3 anterior radii. 



(3) In certain of the Comasteridse, where the left posterior radius is 

 curiously modified, this is often referred to as the posterior radius, the remaining 

 four being collectively termed anterior radii (see fig. 27, p. 69, and p. 111). 



Apical. (1) Aboral or dorsal. 



(2) Applied to the centrodorsal (or cirri), situated at or near the dorsal 

 pole (see fig. 310, p. 269, and pp. 304-306). 



Apical plate. The hypothetical plate covering the center of the dorsal side of the 

 primitive crinoid (compare fig. 71, p. 127, and see pp. 198-200). 



Appendicular skeleton. The skeleton of the division series and arms; the skeleton of 

 the post-radial series. 



Arm bases. The proximal brachials; this term is commonly employed to distin- 

 guish the more or less oblong earlier brachials as distinct from the triangular 

 brachials beyond them (see figs. 30, p. 71, 61a-c, p. 87, 79, p. 132, 94, p. 155, 

 109, p. 175, and 110 p. 176). 



Ann pair. Any two free undivided arms which arise from the same axillary. This 

 term is rarely met with except in reference to 10-armed species, in which each 

 of the post-radial series is sometimes referred to as an arm pair. 



Anns. (1) Strictly speaking, the series of ossicles subsequent to the last straight 

 muscular articulation; or the series of ossicles beginning with the one imme- 

 diately preceding the last synarthry; thus in the Pentametrocrinidae the arms 

 begin with the first ossicle beyond the radials; in the Uintacrinidae they begin 

 with the third ossicle beyond the IBr (costal) axillary; in the remaining conia- 

 tulid families they ordinarily begin with the first segment after the last axillary, 

 except hi the genus Eudiocrinus, in which the third segment beyond the radials 

 is the first arm ossicle. In the recent comatulids the true arms never divide 

 (see figs. 61a-c, p. 87, and pp. 109 [5], 110 [6]). 



(2) While the preceding definition delimits morphologically homologous 

 arms, it is more convenient for practical descriptive purposes to consider the 

 arms as including the entire undivided series of ossicles beyond the last axillary, 

 or beyond the radials hi the Pentametrocrrnidas and in the genus Eudiocrinus 

 (see figs. 1, p. 60, and 2, p. 61, and p. 110 [6]). 



(3) Several authors have considered all the ossicles beyond the radials, 

 no matter how many divisions there may be, and without regard for the type 

 of division, as morphologically comparable arms; this view is inadmissible, for 

 the reason that the radial is an integral part of the series of ossicles following, 

 and is not properly a calyx plate at all. 



