POURTALESIA TANNERI. 



135 



The edge of the actinal groove and the apical system are both placed 

 more towards the anterior extremity than in P. laguncula. On the abactinal 

 side the plates of the ridge formed by the odd posterior interambulacrnm 

 are covered by two to four large primary tubercles and minute miliaries. 

 Fig. 192, (PL 56, fig. S). On the central part of the sides of the test, Fig. 

 193, the primary tubercles are arranged in pairs, with distant miliaries on 

 the rest of the plate. P. Tanneri is also marked for the small number of 

 large primary tubercles found on the sides of the test (PL 56, fig. S). 



On the actinal side the prominent primary tuberculation of the sternum 

 and of the actinal plates of the posterior interambulacra and the anterior 

 ambulacra (PL 56, fig. /) is in marked contrast with the few primary tuber- 

 cles of the actinal face of P. laguncula? The great length of the sternum 

 is also a characteristic feature of P. Tanneri, Fig. 194. The sternum of 

 P. Jeffrey®, is broader anteriorly Fig. 195. 



17 linn. 



Fig. 194. Poubtalesta Tanneri. 



Fig. 195. P. Jeffreysi. 



After Lovi;n. 



An important structural difference between these two species is the 

 existence of a well developed labium in P. laguneula, 2 varying greatly in 

 outline, its narrow edge barely reaching the actinal edge, and the exclusion 

 of the primordial plates of the posterior interambulacra from the actinal sys- 

 tem. In P. Tanneri the labium is excluded from the actinal edge, Figs. 172, 

 173; it is reduced to a diminutive pointed elliptical plate (PL 57, figs. ], ..'), 

 or is represented by a minute plate surrounded by the actinal posterior lat- 

 eral ambulacra! plates. In another specimen it cannot be traced, and the 

 small primordial plates of the posterior ambulacra can be seen at the actinal 

 edge between the long narrow actinal ambulacral plates of the lateral ambu- 

 lacra, Fig. 196 (PL 57, figs. 1, .', 5), while the actinal primordials of the 



1 "Challenger - ' Echinoidea, PI. XXII", fig. 9. 2 Loven, Pourtalesia, PI. VI, figs. 37-40. 



