SPATAGOCYSTIS CHALLEXGERI. 141 



mordial plates of the paired anterior interambulacra are small, wedge-shaped, 

 elongate plates separating at the actinostome the anterior from the posterior 

 pair of ambulacra. The sternum of both is followed by two long episternal 

 plates, larger in P. miranda than in P. Jeffreysi, It is, however, in profile 

 that the great differences in the outline of the test are best seen ; the 

 actinal face of P. miranda is quite flat, compared to its arched outline in P. 

 Jeffreysi. The vertex of P. miranda, Figs. 202, 202 a , coincides with the 

 position of the abactinal system ; in P. Jeffreysi the vertex is placed in the 

 centre of the highly arched abactinal side. The coronal plates carry a 

 greater number of primary tubercles in P. Jeffreysi than in P. miranda, 

 though the odd interambulacrum of the latter is more closely covered 

 by small primaries than in the former. 



The striking differences found in the various groups of species of Pour- 

 talesiag would seem to warrant the splitting up of the genus Pourtalesia 

 into subsections. We might retain the name of the genus, Pourtalesia, for 

 the bottle-shaped types allied to P. miranda, such as P. Tanneri, P. laguncula, 

 P. Jeffreysi, and form a section of the genus for the elongate P. phiale and 

 another for the stout-tested P. ceratopyga and P. rosea. P. hispida may yet 

 be found to belong to a special genus. 



Spatagocystis A. Ag. 

 Spatagocystis Challengeri A. Ag. 



Spatagocystis Challengeri A. Ag., Proc. Am. Acad. Vol. XIV, 1879, p. 206. 

 Spatagocystis Challengeri A. Ag., " Challenger " Echinoidea, 1881, p. 141. 



Plates 70 ; 71. 



From a specimen of Spatagocystis Challengeri among the species left in 

 Cambridge I am able to give some additional details which were not suffi- 

 ciently brought out in the "•Challenger" illustrations. A view from the 

 actinal side (PI. 70, fig. /) of a specimen 48 mm. in length shows the plates 

 of the test far better than they are shown from the fragments figured on 

 Plate XXVI a - Challenger" Echinioidea. An enlarged figure of the plates 

 surrounding the actinal system (PI. 70, fig. 4) shows the comma-shaped 

 labium Fig. 206, and on one side what is probably the remnant of a primor- 

 dial plate enclosed in the right posterior ambulacrum, see also Fig. 177 a 



