PLEXECHINUS CINCTUS. 151 



riorly, and gradually lost in the keel of the actinal surface. The anal 

 proboscis is bound by a wide fasciole (PI. 58, figs. 1-J t ) well beyond the 

 posterior edge of the anal system (PI. 59, figs. ,.\ .;) and sloping toward 

 the anterior extremity. The fasciole stands out very prominently, as 

 the ambulacral plates both on the actinal side and the side of the test 

 Hanking the posterior extremity are bare, while the odd interambulacral 

 plates enclosed by the fasciole are closely tuberculated with small primaries 

 and minute miliaries, as is also the whole abactinal part of the test. The 

 actinal plates of both the ambulacral and interambulacral areas adjoining 

 the ambitus as well as the plastron are covered with larger primary 

 tubercles, while the posterior zone of the posterior ambulacra as well 

 as the actinal plates of the ambulacra are bare or carry but few miliaries 

 (PI. 59, fig. /). 



The actinal system is circular, strengthened by an outer row of plates, 

 largest on the anterior edge, gradually becoming smaller towards the pos- 

 terior edge. The central part of the actinal system is covered by minute 

 elongated plates radiating from the actinostome (Pis. 58, fig / ; 59, fig. 2). 



The anal system is transversely elliptical (Pis. 58, fig. ;; 511. fig. .<) ; an 

 outer row of larger irregularly shaped plates surrounds the small elongated 

 central plates. 



These specimens are evidently young stages, as no trace of genital 

 openings could be seen, unless one of the openings seen on the large inter- 

 ambulacral madreporic plate in continuation of the odd interambulacrum be 

 a genital pore (PI. 60, figs. 1-.:). 



As in Pourtalesia the ocular plates are not present or could not be 

 traced ; and not only is the bivium separated from the trivium by the abac- 

 tinal plates of the lateral posterior interambulacra and some intercalated 

 plates (Pis. 58, fig. ?; 60, figs. 1,2), but the ambulacra of the trivium are also 

 disconnected by the intrusion of the lateral anterior interambulacra, and 

 those of the bivium by that of the odd posterior interambulacrum, Figs. 217, 

 218. It will be seen that there is great variation in the position of the in- 

 truding interambulacral plates and intercalated plates. In Plate 60, fig. 2, 

 the intruded plates seem to belong without doubt to the odd and posterior 

 lateral interambulacra. while in Plates 58, fig. ? ; 60, fig. 1, the relation 

 of the intruding interambulacral plates and the intercalated ones is by no 

 means so simply traced, reminding us more of the arrangement of the 

 abactinal plates of P. Tanneri and P. laguncula. There seems to be some 



