212 ON THE PERIGNATniC GIBDLE OF THE ECHTNOTDEA. 



Fig. 41. Biadema setosum, Gray. A ridge from behind. 



Fig. 42. A ridge of another specimen, from behind. 



Fig. 43, A ridge showing the encroaching plates 2 on the edge. 



Fig. 44. The suture-face at the side of a ridge between it and a process ; three 

 plates are seen at their edges in the ridge, and thus the plate at the 

 actinal surface next to the peristome, and which would be counted as 

 plate 1, is really plate 3. It is marked " a," and the lamellar condi- 

 tion of the suturing is very distinct. 



Fig. 45. The suture-face, showing the lamellar condition and the great projec- 

 tion of the peristomial part or face of the ridge. 



Fig. 46. Outline of the processes and wide arch. 



Fig. 47. Side view of junction of a ridge and process. 



Fig. 48. Echinothrix Desori, Peters. The ridge of an interradium from behind, 

 the line " a* " denotes the commencement of the rise of the ridge. 



The black markings are in the positions of the usual depressions on 

 the plates. 



Fig, 49. A magnified view of the sutures at the median line of the figure 43. 



Fig. 50. Laganum depressum, Lesson. View of part of the test, lower part, 

 seen from within at the edge of the peristome, "^"is part of an 

 ambulacrum, and "in" is the whole of the first interradial plate on 

 which is the small upward projecting homologue of a ridge. The 

 plate is single, and is succeeded by plates 2 of both zones. 



Fig. 51. Side view of interradial plate and ridge. 



Fig. 52. One of the posterior projections (ridges). 



Fig. 53. Clypeaster {Echinanthus, A. Ag.) rosaceus, Linn. Internal view of the 

 actinal part of the test at the peristome. " A " is part of the ambula- 

 crum ; " in " is the single interradial plate ; "^ " is the base of a process. 



Fig. 54. Processes of neighbouring ambulacra with their bases separated by a 

 narrow interradial plate. 



Fig. 55. A view of a process of an ambulacrum, seen from the median ambulacral 

 line outwards. The small pairs of pores are on the flank of the 

 process. 



Fig. 56. Clypeaster humilis, A. Agass. The actinal part of a test seen from 

 within and at the peristome. The central single plate is the inter- 

 radial, and it is narrowest at the peristome " in." The ambulacra on 

 either side are partly shown, and the bases of the processes of two 

 neighbouring ambulacra are marked "p." 

 Fig. 57. An oblique view of the same surface and specimen, showing the pro- 

 cesses and the intervening interradial plate. 

 Fig. 68. View from behind ; " in " is the posterior edge of the stout interradial 

 plate, and the plates on either side are ambulacral. 



