POURTALESI.E. 



127 



Ala iVa 



which is narrow, small, elongate (Pis. 70, fig. 4; 71, fig. G,, by large com- 

 posite double plates of the lateral anterior interambulacra and by the 

 second rows of plates of the lateral poste- 

 rior ambulacra extending along the ster- 

 num, which also separate the actinal plates 

 of tlie lateral posterior ambulacra from the 

 actinal plates. 



The position of the anal system on the 

 actinal side in Echinocrepis and Spatago- 

 cystis materially alters the proportion of 

 the actinal plates of the odd interambu- 

 lacrum from those of Pourtalesia proper 

 and Plexechinus, in which the anus is in 

 a sunken groove at the base of a well 

 developed anal snout, round which a fas- 

 cicle is developed. Compare Figs. 167, 

 175 to Figs. 171, 174. 



The shape of the actinal system of 

 Pourtalesia Tanneri, Fig. 176, differs con- 

 siderably from that of P. Jeffreyd^ and of 

 P. cannata? It is round in the former 

 (PI. 57, fig. '5"), somewhat rectangular in 



Fig. 175. 



ft 5 a 



48 mm. 



Spatagocystis Ciiali.exgeri. 



ll'i nun. 



Fig. 176. Pourtalesia Tan.veri. 



91 mm. 



Fig. 177. Echinocuepis setigkra. 



the second, and oval in the third. In E. sdigera the actinal system is pear 

 shaped, Fig. 177, and pointed abactinally in >ip. Challeugeri, Fig. 177"*. 



1 Loven, Tourtalesia, PI. IV, fig. 18. ^ »» Cliallenger" Echinoidou. PI. XXVIII», fip. 9. 



