13 



several peculiarities in their form, as also the insertion of single calcareous plates between 

 them, to which nothing analogous is to be observed in the other vertebrae, and which are 

 destined to attach them still more firmly together. 



If we once more \iew the bucal ring from the lower side (fig. 2 & 5) it will 

 be easily perceived that the radial or depressed spaces make the interior continuation of 

 the arabulacral furrows of the arms, and consequently are formed in the middle by the am- 

 bulacra! plates of the two vertebrae mentioned; while the elevated or interradial spaces are 

 formed by the contact of the adambulacral plates of 2 pairs of adjacent vertebrae. If we 

 then turn the bucal ring, and view it from above (fig. 1 & 4) we shall observe, on that part 

 of it which lies just over the ventral furrow, the dorsal, convex part of the ambulacral plates 

 (a a) of the 2 vertebrae in contact, continuing the dorsal ridge of the skeleton of the arms 

 inwards. On each side of this raised part there are (at the extreme outside) 3 deep cavities 

 which when closely examined ai'e found to extend through the whole thickness of the bucal 

 ring, and appear on the lower side as the before-mentioned holes, situated in pairs in the 

 ventral furrow, for the insertion of the water-feet. lu this manner the double series of holes 

 or ambulacral pores that perforate the skeleton of the arm in its whole length, is thus like- 

 wise continued inwards on the bucal ring. Inside of these holes, the ambulacral plates (a') 

 of the interior vertebrae come on each side in contact with a peculiar plate (m) to which 

 there is nothing corresponding in the other vertebrte, and which we may call the parietal- 

 plate; as it contributes in an essential degree to form the interior wall of the bucal ring. 

 It comes here in contact (see fig. 7) with the corresponding plate on the adjacent vertebrae, 

 while these 2 parietal-plates are separated above by a 3"" equally peculiar single plate (k) 

 wedged in between them, which plate we have previously denominated the wedge-plate. 

 With the exterior end of this plate (which lies just in the angle between 2 arms) there are 

 again connected 2 small plates (r) belonging to 2 adjacent vertebrae of the exterior set, each 

 articulated with one of the interior dorsal marginal plates situated on 2 adjacent arms; 

 for which reason we may also regard those small plates observed on the exterior side of 

 the bucal ring as corresponding to the dorsal marginal plates on the disc of other star- 

 fishes. 



The bucal ring is thus in a 10-armed specimen composed of 20 complete ambulacral 

 vertebrae; 30 peculiar plates inserted between the interior vertebrae, and 20 dorsal marginal 

 plates. The individual plates of which the bucal ring is composed, are therefore as follow: 



