04 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



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

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

 the })ascs 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 difl'erentiate the radii on either side of 

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

 posterior and 3 anterior radii. 



(3) In certain of the Comasteridse, where the loft 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 crinoitl (compare fig. 71, p. 127, and see pp. 198-200). 



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

 the post-radial series. 



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

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

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

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



Arm pair. — jVny two free uncUvided arms which arise from the same axillary. This 

 terra is rarely met with except in reference to 10-armed species, in wliich each 

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



Arm^. — (1) Strictly speaking, the series of ossicles subsequent to the last straight 

 muscular articulation; or the scries of ossicles begimiing with the one imme- 

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

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

 with the third ossicle beyond the IBr (costal) axillary; in the remainmg coma- 

 tulid families they ordinar'dy begin with the firet segment after the last axillary, 

 except in the genus Eudiocrinus, in which the third segment beyonil the radials 

 is the first arm ossicle. In the recent comatulitls the tnie arms never divide 

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



(2) While the preceding defuiition dehmits 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 in the Pentametrocrinidaj and in the genus Eudiocriniis 

 (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 plato at all. 



