Plates, 



14 



Exterior vertebrae. 



Ambulacral plates . 

 Adambulacral plates 

 Parietal plates . . 

 Wedge plates . . . 

 Dorsal marginal plates 



70 



Together 130 single plates. 



+ 



60 



The more particular relation of the 2 sets of vertebrae which enter into the composi- 

 tion of the bucal ring, and of the single calcareous plates belonging to and connecting them, 

 is as follows. 



The exterior vertebrae which, as already noticed, should properly be considered as 

 belonging to the arms (although they always remain fixed to the skeleton of the disc when 

 the arms are detached) are considerably shorter and lower than the interior vertebras 

 (comp. fig. 10). In their structure they correspond very closely with the interior vertebra? 

 of the arms, with which they are to a certain degree movably connected in nearly their 

 whole height, The articulating surfaces which appear distinctly in every detached disc on 

 the exterior side (see Tab. I, fig. 6, Tab. Ill, fig 1) exhibit therefore also in their form and 

 arrangement the most exact conformity with those on the adoral side of the interior vertebras 

 of the arms (Tab. IV, fig. 20). It is easy to distinguish (see also Tab. IV, fig. 6) the strongly 

 convex central part (a 2 ), formed by the 2 ambulacral plates uniting in the middle, with its 

 2 large vertical articulating surfaces. The side parts inclining outwards and downwards from 

 this central part, form also here the boundaries between 2 pairs of ambulacral pores, of 

 which the interior pair belongs to the disc; while the exterior pair is destined for the attach- 

 ment of the 1 st pair of water-feet of the arm. At the end of these side parts lie the strongly 

 developed adambulacral plates (ad 2 ) which exhibit a large, somewhat inclined, roundly tri- 

 angular smooth articulating surface, whereby these plates are articulated with the adambu- 

 lacral plates of the l sl vertebra of the arm. They are considerably wider than the latter, 

 and are in immediate contact with the nearest adambulacral plates of the 2 adjacent vertebra?, 

 with which they are firmly connected by suture. Above the adambulacral plates there are 

 lastly the before-mentioned dorsal marginal plates (rj situated one on each side furthest 

 outwards, and exhibiting a small semilunar articulator}- surface, whereby these plates are 

 articulated with the corresponding plates on the I s ' joint of the arm. These plates also are, 

 at their interior extremity, in immediate contact with the nearest dorsal marginal plates of 



