CALAMOCRIN I 8 DIOMED I 29 



tlic main stem of the arm arc quite Irregular in their appearance, which 

 depends greatly upon the thickness <>r the cutis which conceals the calca- 

 reous plates forming the skeleton of the side or covering plate- (Plate VII 



Figs. 1, -. IT. and Plate VIII. Fig. 11). tyear the base of the I I groove of 



the arm, close to its junction with the ilisk food groove, the covering plates 

 are most irregular in outline (Plate VII. Figs. I. 2), bul when pressed 

 together form an irregular pavement completely closing it. Some of these 

 covering plates are more or less spiny. The Bide plates themselves are most 

 irregular in shape, and are either digitiform plates, like those of Plate X 

 Figs. L-5, or lamelliform, like those of Plate Nil. Figs. :j. 13. having in addi- 

 tion comparatively large lateral spines, as seen in Plate VII. Figs. 1, 2. 



As we pass from the base of the arm towards the extremity, both the 

 side and the covering plates become more regular in shape, until we have 

 one or two lateral plates supporting a single covering plate. The passage 

 from the irregular plating formed by the covering plates at the base of the 

 arms into a regular alternating arrangement is often quite sudden, and in 

 the same parts of adjoining arms the covering and side plates appear very 

 differently developed. Compare, for instance, the condition of these plates 

 in Plate VII. Fig. 9, and Plate VIII. Fig. 1, or Plate VII. Fig. 14. In general, 

 however, the succession represented in Plate VII. Fig. 1, Plate VII. Figs. 

 4, 5, Plate VII. Figs. 6, 7, Plate VII. Figs. 10, 11, 12. Plate VII. Figs. 15, 

 16, shows the gradual passage of the lateral and covering plates, first form- 

 ing a wall of irregular side plates supporting irregular covering plates, to 

 side plates and covering plates which lose gradually their varying outlines 

 and projecting spines, and become little by little side plates of regular 

 shape, with spines passing into knobs or disappearing entirely, and cover- 

 ing plates alternating with the greatest regularity and forming a beautiful 

 plating of triangular reticulated network. See Plate VIII. Figs. 1-5. 



The stoutness of the side and covering plates diminishes rapidly as we 

 pass from the disk or from the base of the arms towards their extremity. 

 Those of the disk and proximal part of the arms are comparatively stout, 

 often thicker and more solid than the adjoining perforated perisomatic 

 plates; and the side plates especially are greatly strengthened by vertical 

 ribs or by lateral spines, even after the covering plates have become com- 

 paratively attenuated and consist of a very open meshwork limestone, with 

 circular or elliptical meshes and huge projecting terminal spokes (Plate 

 VIII. Figs. 2-4). 



