MONOGRAPH OF THE EXISTING CEINOIDS. 95 



0. 



Oblique mwscular articulation. — See Articulations Ah. 



Opposing spine. — The &\)mo, ridge, or projection on the dorsal side of the penultimate 

 cirrus segment; the last dorsal spine (see figs. 4, p 63, and 314-318, p. 273, and 

 pp. 279-282). 



Oral. — Situated near the border of the disk, but not onits surface (see figs. l,p.60, 

 and 6, p. 63, and pp. 112-113). 



Oral pinnules. — The pinnules of the proximal part of the arm which do not bear 

 gonads, and usually do not possess ambulacra (see figs. 1, p. 60, and 6, p. 63, 

 and pp. 112-113). 



Oral surface. — See Adoral. 



Orals. — Large more or less triangular plates forming a circlet on the disk about the 

 mouth; they are interradial m position and are developed above the basals, 

 from which they may be separated b}' mterradials; though well developed in 

 the young of all comatuUds, they are in almost all cases entirely resorbed before 

 the adult stage is reached. The orals probably correspond to the teeth of 

 echmoids (see figs. 117, p. 183, 407-413, p. 317, 530, pi. 2, 532, 533, pi. 3, and 

 542, 544, 547, 548, pi. 4, and pp. 340-341). 



Orientation. — Two methods have been employed to designate the various radii 

 and interradii of the comatuUds: 



(1) The animal is placed with the dorsal side down, and the disk upper- 

 most; the different rays are now distinguished as a, the Anterior; b, the Left 

 anterior; c, the Right anterior; d, the Left posterior; and, e, the Right posterior 

 (see fig. 22, p. 69, and pp. 110-111); the interradial areas being known as a, the 

 Left anterior; b, the Right anterior; c, the Left lateral; d, the Right lateral; and 

 e, the Posterior. 



(2) The annual is placed in the same position; the different rays arc 

 distinguished as Ray A (anterior), Ray B, Ray C, Ray D, and Ray E, counting 

 from left to right foUowmg the hands of the clock; the prlmarj- derivatives of 

 the rays (the IIBr series and their derivatives) are represented by inferior 

 numbers, these being, beginning with the left-hand branch of the anterior ray, 

 A„ A,, Bi, B,, C,, C,, Dj, D,, Ej, and E,; follo\vhig tliis system the riiterradii 

 are called Interradius A-B, Interradhis B-C, Interradius C-D (the posterior), 

 Interradius D-E, and Interradius E-A. 



(3) In those comasterids in which the mouth is interradial (situated in 

 the right anterior interradius, or interradius A-B) tlie left jiosterior ray (D) 

 which is opposite to it often becomes greatly modified, resulting in a swmging 

 of the true anteroposterior axis from its original jiosition througli an arc of 36°, 

 so that it traverses the center of the right anterior interradius (A-B) and tlie 

 center of the left posterior arm; in this case the right anterior interradius is 

 sometimes spoken of as the Anterior interradius, and the left posterior arm as 

 the Posterior arm, with a corresponding change hi regard to all the other radii 

 and interradii (see figs. 27, 28, p. 69). 



