PHORMOSOMA ]I ISIM DIM. 97 



a view from the interior of the same interambulacra (I'l. 32, fig. /) the 

 split and subdivided plates of the ambiilacral area are specially marked. 

 The subdivision of the interambulacral plates is better shown from tlie 

 abactinal side (PI. 32, fig. 1), though it is very striking also in an interior 

 view of the test of a specimen 140 mm. in diameter (PI. 34, fig. /;. The 

 characteristic lapping of the interambulacral plates of the Echinothuriae is 

 well shown. 



The breaking up of the plates of a part of the right posterior interarnbu- 

 lacrum and the adjoining right })osterior ambulacrum of a specimen 203 mm. 

 in diameter is shown on Plate 45, figs. J3, J^. The outline of the original 

 plates is still very distinct, while the interior of each of the larger plate.s 

 is broken up into a number of smaller plates apparently without any regu- 

 larity. In the actinal plates of the left anterior ambulacrum (PI. 45, fig. 8) 

 the first trace of the breaking up of the anibulacral plates is shown from 

 the exterior, and in Plate 45, fig. 9, the same plates are seen from the 

 interior. 



Tn Plate 32, fig. 2 the reverse imbrication of the ambulacral plates is not 

 as distinctly seen as in the interior view of a somewhat smaller specimen 

 (PI. 34, fig. i), which also shows the crowding past the auricles, under their 

 arches, of the ambulacral plates as they pass over onto the actinal area. The 

 same figures also show the imbrication of the plates of the actinal system, 

 that is always best seen from the interior, where the regular tile-like 

 arrangement of the successive rows of buccal plates is very prominent (Pis. 

 33, fig. 3; 34, fig. ,5). Compare these figures to those of the actinal system 

 seen from the exterior of the test (Pis. 33, fig. 1 ; 34, fig. 2). 



The extent of the imbrication of the buccal plates is shown on Plate 45, 

 fig. 10, which represents the successive actinal plates of the right zone 

 of the odd ambulacrum. The oldest plate is at the bottom of the 

 figure ; the shaded and tuberculated parts of the figure represent the 

 portion not covered by the lapping of the two younger plates above it.^ 

 The third and fourth plates are also figured from the opposite side ; the 

 youngest plate is shown from the interior and in profile (PI. 45, fig. //). and 

 one of the plates of the left zone of the left posterior ambulacrum is figured 

 on PI. 45, fig. J2. 



The primary tubercles near the ambitus are usually crenulated, and the 

 scrobicular area is frequently slightly raised (PI. 45, fig. IS), the edge 



1 One of its own ambulacrum, the other of the one next to it. 



7 



