PERIGNATHIO GIEDLE OP THE ECHINOIDEA. 211 



Fig. 18. View of a ridge from behind ; cp^ is the line of base-level of the ridge. 



Fig. 19. The processes forming an arch in a small specimen. 



Fig. 20. Processes of a smaller specimen, not yet joined and low. 



Fig. 21. Processes in a very small specimen ; they are mere knobs situated on 



the poriferous zones of the ambulacra. 

 Fig. 22. Side view of a ridge, showing the presence of two plates. 

 Fig. 23. The view of the first three plates of one zone of the ambulacrum 



of the specimen figured in fig. 21, seen from within the test ; the 



ovoid base of the process is indicated "p ; " the median line of the 



ambulacrum is where the figures are placed, and the interradium 



is marked "in. ' 

 Fig. 24. The ridge of an interradium of a large specimen ; " a* " is the line 



where the ridge begins to rise from the upper surface of the interradium. 

 Fig. 25. Psammechinus miliaris. A ridge seen obliquely and two processes of 



an ambulacrum forming an arch. 

 Fig. 26. Processes not joined above ; see the line of suture between them and 



the ridge on one side. 

 Fig. 27. A ridge, peristomial face. 

 Fig. 28. A ridge from behind and part of the interradium ; a* is the line above 



which the ridge rises from the iuterradial level. 

 Fig. 29. Side view of a ridge, showing sutural lines. 

 Fig. 30. Back view of the base of a process, showing the pairs of pores and 



those wlaieh are in plates sutured to the ambulacro-interradial suture. 

 Fig. 31. A broken process, showing tentacular canals within. 



Plate XXXI. 



Fig. 32. Strongyloceiitrotus lividios, Lmk., sp. View of the back of a ridge : the 

 numbers refer to plates and the letters to zones of the interradium. 

 The line a^ is that of the level of the base of the ridge, and a** refers 

 to the level of a base. See fig. 42. 



Fig. 33. A ridge from behind, showing the presence of two no. 1 plates at the 

 free edge, instead of only one as in fig. 32. 



Fig. 31. A ridge, back view, showing almost complete exclusion of one of the 

 first plates at the edge. 



Fig. 35. Eohinometra luoimter, Leske. A typical arrangement of the plates of 

 a ridge, seen from behind. 



Fig. 36. Outline of the capped processes of Eohinometra lucunter. 



Fig. 37. Echinometra subcmgularis, Leske, sp. The capped processes. 



Fig. 38. StrongylocentTotus lividus. A process seen from behind and side- 

 ways, showing the large and small pores, and that the plates of pairs 

 4 and 5 reach the ambulacro-interradial suture. 



Fig. 39. The ridge has been separated at the suture from the side of a process, 

 and " y " is the suture-face on the side of the process ; " ^ " is a line 

 which marks the base of the process ; and the numerals 4 and 5 are 

 placed over the tops of the interradial ends of the corresponding 

 ambulacral plates. 



Fig. 40. A front or peristomial view of a process, .showing the limits of plates 

 forming the base of it ; " a? " is the line of limitation. 



17* 



