107 



Fig. 5. The same seen from below. 

 Fig. 6. The same viewed from the exterior side. 

 Fig. 7. The same viewed from the interior side. 



Fig. 8. The same viewed obliquely from above and from the interior side, the better 

 to see the furrow for the circular ambulacral vessel, with one of the holes through which 

 the ambulacral vessel passes. 



Fig. 9. A piece of the oral ring, from which the exterior set of vertebrae are removed. 

 Fig. 10. Transverse section of the oral ring in the middle of a radial space. 

 Fig. 11. Transverse section of the oral ring in the middle of an interradial space. 

 The following indications serve for the last 8 figures: 

 a l , interior ambulacral plates. 

 a 2 , exterior ambulacral plates. 

 ad l , interior adambulacral plates. 

 ad 3 , exterior adambulacral plates. 



f, furrow for the radial ambulacral vessel. 

 &, wedge-plates. 



I, interior cavity bounded by the wedge-plates and the parietal plates. 

 m, parietal plates. 



o J , apertures for the interior water-feet of the disc. 

 o 2 , apertures for the exterior water-feet of the disc. 

 r, dorsal marginal plates. 

 x, furrow for the circular ambulacral vessel. 



/, the circular rim, projecting from the interior wall of the oral ring, which below forms 

 a boundary for the circular ambulacral vessel. 

 Fig. 12. A wedge-plate isolated, viewed from the exterior side. 

 Fig. 13. The same, viewed from the interior side. 

 fig. 14. The same, viewed in profile. 



Fig. 15. The base of the skeleton of an arm, viewed from above, somewhat magni- 

 fied: a a, the dorsal ridge formed by the interior part of the ambulacral plates; & &, the 

 lateral parts of the ambulacral plates; ad, adambulacral plates; c, the 2 connate dorsal 

 marginal plates at the base of the arm; r, rudimentary marginal plates for attachment of 

 marginal spines; o, holes for the water-feet. 



Fig. 16. The same seen from below: f, the furrow for the ambulacral vessel; 

 p l , marginal spines; p 2 , p 3 , furrow-spines; the other letters as in the preceding figure. 



Fig. 17. The same, viewed from the left side: a, ambulacral plates; ad, adambu- 

 lacral plates; 6, the lateral parts of the ambulacral plates; r\ r z , the 2 connate dorsal 

 marginal plates at the base. 



Fig. 18. Base of an ambulacral skeleton split along the middle and viewed from 



the surface of fracture: f, furrow for the ambulacral vessel. 



14* 



