150 



PANAMIC DEEP SEA ECHINI. 



Outside view. 



Fig. 219. Urechijtus Naresianus 



After Lov£n 



The labium is separated from the sternum by the posterior zone of 

 the posterior ambulacra (Pis. 58, fig. 1 ; 59, figs. 1, ,:). As in Urechinus the 



primordial plates are all in contact with the 

 actinal system (Pis. 58, fig. 1 ; 59, figs. 1, 3). 

 It is interesting to trace the changes be- 

 tween Pourtalesia proper with its bottle- 

 shaped outline, deeply sunken actinal and 

 anal grooves, its well developed anal probos- 

 cis, and such a type as Plexechinus, in which 

 the Pourtalesian features have almost disap- 

 peared, to pass into a more Ananchytid type 

 represented by Urechinus and Cystechinus. 

 In the further development the rudimentary 

 phyllodes ami labium become specialized in 

 Genicopatagus, Argopatagus, and Homolam- 

 pas. Next Ananchytid petals like those of Paleopneustes, Linopneustes, 

 lead us gradually to the petaloid type of the recent Spatangoids. 



The recent Ananchytid Spatangoids might thus naturally be subdivided 

 into three families: the Urechinidse, with such types as Urechinus, Cyste- 

 chinus, Pilematechinus, and Calymne; a second group containing the 

 Paleopneustida% containing genera like Linopneustes and Paleopneustes, — 

 all types more closely allied to the recent Spatangoids ; with perhaps a third 

 group containing Argopatagus, Genicopatagus, Homolampas, and Palaeo 

 tropus, though for the present the third group may remain united with 

 the Paleopneustidce. 



Plexechinus cinctus A. Ag. 



Plexechinus cinctus A. Ag., Bull. M. ('. Z. 18'.»8, XXXI I, No. 5, p. 76. PI. VIII, figs. 3, 4. 



Plates 55, figs. 3-5; 58. figs. 1-4 ; 59; GO, figs. 1-3. 



Only two slightly broken specimens of this interesting species were 

 collected, at Station 3424. The smaller of the two specimens measured 

 21 mm. in length, 15.5 mm. in width at the anterior part of the posterior 

 lateral interambulacra, and 11 mm. in height immediately in front of the 

 anal system. Seen in profile (Pis. 55, fig. 5; 58, fig. •-') the test slopes 

 very gradually from the highest point in front of the anal system towards 

 the rounded anterior extremity. The posterior extremity is scooped out for 

 the anal system above the broad anal proboscis, winch is rounded poste- 



