CALAMOCKINUS DIOMED.Ii. 31 



iToove of the arms near their base adjoining the disk is close and indis- 

 tinct, similar to that of the corresponding side plates (Plate X. Figs. 1-5). 

 As we proceed toward the extremity of the arms, the original elliptical 

 gronndwork becomes more apparent (Plate VII. Fig. 9, Plate VIII. Fig. 1), 

 and the covering plates assume a more triangular outline, with a slightly 

 re-entering base and rounded apex. The original elliptical reticulation 

 forms the network of the base of the covering plates, and fi'om this radiate 

 spokes connected laterally by ti'ansverse rods forming rectangular or irreg- 

 ularly shaped cells of considerable size compared with those of the base 

 of the covering plates (Plate VII. Fig. 9, Plate VIII. Figs. 2-5, Plate XI. 

 Fig. 2), the radiating spokes often projecting far beyond the outline of 

 the covering plates. See also Plate XL Figs. 8, 9, Plate XII. Fig. 7. 



The covering plates are also frequently surmounted with spines (Plate 

 VII. Figs. 1, 2, 5). These spines ai-e formed like the lateral spines of the 

 side plates by the formation of a secondary reticulation. These spines may 

 be at the surface of the covering plate or on the fan-shaped extremity 

 (Plate XI. Fig. 2) ; or they may be lateral spines of the wing-like projections 

 formed at the base of the covering plates (Plate X. Fig. 9), which form 

 a sort of saddle (Plate X. Figs. 9, II) by which it is roughly articulated 

 to the adjoining side plates. This saddle-like process and its formation are 

 well shown in the structure of some of the covering plates from pinnules 

 taken near the base and the middle of the arm. Figure II of Plate XI. 

 is a covering plate seen from above, showing its regular original elliptical 

 reticulation ; Figure 12 shows a similar covering plate from the inside, Avith 

 the beginning of the secondary reticulation which is to form the spurs to 

 articulate with the adjacent side plates ; Figure 13 is a side view of one 

 of the covering plates showing the wings or spurs connecting it Avith the 

 adjacent side plates ; and Figure 3 is one of the covering plates of a pinnule, 

 with a large open irregular reticulation at the extremity. 



The mode of formation of the spurs and wings connecting the covering 

 plates to the side plates is perhaps best seen in the figures of a covering 

 plate taken from the groove of a pinnule close to the base of the arms. 

 Figures 9 and 10 of Plate X. are the extremity and the saddle-like base of 

 one of the covering plates. The secondary reticulation, to increase the 

 thickness of the plate, or to form spines, has begun to form at the base of 

 the spines (Plate X. Fig. 9) and on the sides of the extremity (Plate X. 

 Fig. 10) ; otherwise the original reticulation is still quite distinct. In Fig- 



