142 



PANAMIC DEEP SEA ECHINI. 



(PI. 71, figs. 1, G). The actinal plates of the anterior lateral ambulacra 

 are probably primordial plates, Fig. 162. The labium (Fig. 162) is, as in 

 Pourtalesiae, separated from the sternum (PI. TO, figs. 1, 2, 4 ; 71, fig. 6) 

 by the plates of the anterior zone of the lateral posterior interambulacra 

 and by the pair of plates of the lateral posterior ambulacra, which are 

 separated from the actinal plates by the interambulacral zone. The ster- 

 num is remarkable for its small size (Fig. 175) and prominent tubercu- 

 lation (Pis. 70, figs, i, ^,4, ^ ; 71, fig. '>)^ which forms the extension of the 

 actinal keel on the line of the odd posterior interambulacrum. Seen from 



ocp 



I a. 



48 mm. 



Fia. 200. Spatagocystis Challesgebi. 



ocp, 



5 '^ h\ 



4S mill. 



Fig. 207. Spatagocystis Challeiiqerl 



the interior of the test, Fig. 207 (PI. 70, fig. 5), one is struck with the great 

 uniformity in the size of the coronal plates forming the bivium on the 

 abactinal part of the test. Above the ambitus the posterior lateral am- 

 bulacral plates are much elongated. Similarly the plates of the actinal 

 side, except those surrounding the anal system and composing the odd 

 interambulacrum, are quite uniform in size. The odd interambulacrum is 

 reduced on the actinal side, beyond the anal system, to five small pairs of 

 plates on the actinal edge of the anal system, to the small sternum, and the 

 still smaller labium (PI. 70, figs. /, :', o). The position and shape of the 

 plates of the odd interambulacrum and round the anal system is well seen 

 in an interior view of the anal system. Fig. 208, extending from the 

 ambitus to the sternum and beyond (PI. 71, fig. 5). 



