POURTALESIiE. 



123 



mens of 13, Fig. IGl-"^, and 18 mm. (PI. 80, fig. /) ; in the liitter the primor- 

 dial plates of the right lateral interambulacra are excluded from it, which h 

 not the case in the older specimen of 56 mm. (PI. 80, fig. ,?), Fig. 163. 



In the odd interanibiilacrum of 

 Plexechinus a well developed hexag- 

 onal sternum consisting of a single 

 plate is separated from a small lan- 

 ceolate labium by the second plates 

 of the posterior lateral ambulacra ; 

 this sternum is only an emphasized 

 single second plate such as exists in 

 the other interambulacral areas of 

 the genus. 



The sternum of Cystechinus Lo- 

 veni (Pis. 77, fig. 1 ; 78, fig. j), Fig. 

 160, is connected with the labium, 

 but otherwise represents a somewhat 

 specialized single second interambu- 

 lacral plate of the other interambu- 

 lacral areas. 



In Cystechiniis Wi/villii (PI. 80, figs, 

 i, 2) the sternum holds the same relations to the labium and to the single 

 interambulacral plates which follow the primordials. Fig. 161% as in C. 



5 



48 mm. 



Fig. 162. Spatagocystis Ciiallexgeri. 



^y^ 



1^^ 



Fig. 161». Cystechinus Wyvillii. 



50 mm. 



Fig. 163. Cystechims Wyvili.ii. 



Loveni. In the younger specimens the sternum holds to the labium tlie 

 same relation which the second plate holds to the primordial interambu- 

 lacral plates. 



