122 



PANAMIC DEEP SEA ECHINI. 



extremity, develop to an extraordinary extent in such recent Spatangoids 

 as Metalia, Schizaster, Desoria, Brissopsis, and others. It is interesting to 

 note that already in Ilemiaster and Micraster of the Chalk we find the same 

 excessive development of the sternum; while in Holaster the labium is 



Ik 



Fig. 159. Urechinus Nakesianus. 



Attee Lov£n. 



83 mm. 



Fig. 160. Cystechixus Lovexi. 



followed by a sternum made up of a single rectangular plate as in Anan- 



chytes, though the other actinal primordial interambulacral plates are 



followed by two plates. 



A single plate follows the interambulacral primordial plates not only 



in Urechinus but also in Plexechinus (Pis. 58, fig. 1 ; 59, fig. 3), in C^st- 



echinus Loveni (Pis. 77, fig. 1 ; 78, 

 fig. 3), Figs. 160, 161, in C. WyvilJu 

 (PI. 80, fig. 2), Fig. 161% and in P. 

 Bathbuni {V\s. 83, fig. /; 85, figs. 

 /, ?), Fig. 164. In Spatagocystis 

 (PI. 71, fig. G) the primordials are 

 excluded from the actinal system 

 in the lateral posterior ambulacra. 

 Fig. 162. The small labium is 



separated from the sternum by the second and third set of plates of the 



posterior lateral ambulacra, and the second set of the lateral interambulacra. 



Fig. 175. 



In Cystechinufi Wyvillii there is also some difference in the arrangement 



of the plates around the actinal system between older and younger speci- 



85 mm. V 



Fig. 101. Cystechinus Loveni. 



