15 



the adjacent vertebrae and immovably connected with the same, as also with the extremity 

 of the wedge-plate (k) belonging to the interior vertebrae. 



In the interior set of vertebrae, which are properly those that belong specially to 

 the disc, we can likewise distinguish (see fig. 9) a pair of ambulacral plates (a 1 ) and a pair 

 of adambulacral plates (ad 1 ); while there are no proper dorsal marginal plates. In place 

 of the latter, there are between each vertebra and its neighbor inserted 3 peculiar plates 

 which we have denominated the parietal plates (m) and the wedge-plate (k). 



The ambulacral plates are not only considerably longer (see fig. 10) but also higher 

 and broader than on the exterior vertebras ; while the adambulacral plates (see fig. 5 ad ') 

 are much narrower, and terminate in a somewhat unevenly truncated edge, which projects 

 freely under the thickened margin (y) and to which are attached the spines radiating to- 

 wards the mouth (the bucal spines) (see fig. 2). Each of the ambulacral plates is, as before- 

 mentioned, connected at one extremity by distinct suture with a so-called parietal plate. 

 This is (see fig. 4, 6, 7, 8 m) quite narrow above, and here contributes to form the boundary 

 of the interior pair of ambulacral pores, but is considerably enlarged on the interior 

 side of the bucal ring; so that these plates contribute essentially at this part to form its 

 interior wall, (see fig. 7), as also the above mentioned inwardly salient margin (y), which 

 makes the lower boundary of the circular semi-canal for the ambulacral vessel of the disc, 

 appears to be formed exclusively by these plates. On the other hand the ambulacral plates 

 themselves (a *) taper here rapidly, as the sutures between them and the parietal plates con- 

 verge downwards towards the holes, situated in the semi- canal mentioned, for the radial 

 ambulacral vessels (0). Each parietal plate comes close up on the interior wall otthe bucal 

 ring to the adjoining one on the adjacent vertebra, yet without being immediately connected 

 with the same; as there is always between them a very narrow crevice leading into a rather 

 spacious cavity in the interior of the bucal ring (see fig. 11, 1). These pariietal plates are 

 however above separated from each other by the insertion between them of a single pecu- 

 liar plate (k), which may in a manner be said to continue or terminate the series of dorsal 

 marginal plates inwardly. This plate, which we have called the wedge-plate, is (see fig. 12, 

 13, 14) quite narrow, in the middle somewhat constricted and nearly cylindrical; while the 

 upper part is enlarged to a tubercular prominence with corners drawn out on each side. Its 

 exterior surface is smooth and convex, while its interior surface is somewhat uneven, and 

 slopes from each side upwards towards the middle (see fig. 13). It is wedged in with its 

 upper part between 2 parietal plates, so as to extend round the dorsal edge of the bucal ring, 

 and a little way down on the inner surface (see fig. 7, 8) ; its lower extremity is truncated, 

 and on each side connected by sutures with the lower lateral extremity of an ambulacral 

 plate of the interior set of vertebrae; while it connects itself in the middle with 2 of the 

 dorsal marginal plates belonging to the exterior set of vertebrae. 



