PALEOPNEUSTES AND LINOPNEUSTES. 183 



270, the labium is quite broad. In P. cristatus, the third plate occupies 

 nearly its whole flank, Fig. 269, while in P. hystrix, Figs. 266, 270, in which 

 it is shorter and more arched, the sides of the labium are flanked by the 

 first, second, and third ambulacral plates which are quite small. In Phris- 

 socystis the labium is deeply indented by the first plate, Fig. 271. In 

 Argopatagus, Fig. 272, it resembles somewhat that of Phrissocystis, and is 



110 mm. 1 In hum 135 mm. 



Fig. 268. Fro. 269. Fig. 270. 



LINOPNEUSTES I.oNoIKPINX'S. PALEOPSTEUSTB3 CRISTATUS. PAI.EOP. HYSTKIX. 



flanked for nearly its whole length by the first and second plates. In 

 Genicopalagus, Fig. 273, the labium is mure like that of the Urechinidce. 



In Linopneustes Murrayi* the labium is small, followed by a long narrow 

 single plate, while in the West Indian Linopneustes the labium is not thus 

 subdivided (PI. 92). Linopneustes excentricus de Meijere 2 has the elongate 

 narrow labium characteristic of the West Indian Linopneustes longispinus 

 (PL 92, figs. A .<). 



The sternum of Plesiozonns hirsutus of M. de Meijere 3 with its short labium 

 is very similar to that of Pakopneustes cristatus (PI. 95). Plesiozonus differs 



120 mm. ? 4S ram, ? 37 mm. 



Fig. 271. Fki. 272. Fig. 273. 



Phrissocystis aculeata. Argopatagus vitreus. Genicopatagi's affinis. 



from Linopneustes and Paleopneustes by its small ambulacral plates alternat- 

 ing near the extremity of the petals with the larger plates. The labium of 

 Paleopneustes spectabilis and of P.fragiUs, as figured by M. de Meijere, 4 recalls 

 more the labium of Linopneustes than of Paleopneustes. 



At the ambitus of Linopneustes (Pis. 92; 93; 94, fig. l), as in Phrisso- 

 cystis (Pis. 88, figs. A ~ ; 90- fig s - I* ■'% there are a number of very narrow 



1 '■ Challenger " Eehinoidea, PI. XXV, figs. 2, 7. 8 Id. PI. XXIT, fig. 441. 



- " Siboga " Eehinoidea, PI. XXII, fig. 454. 4 Id. PL XXI, figs. 422, 427. 



