286 PEOF. p. M. Duncan's eetision of the 



of the riglit posterior interradium is from the union of the second 

 and third plates of zone "5." This is unusual amongst the 

 Spatangoidea. 



In P. Jeffreysi the labrum is very small and is separated from 

 the sternum by the disjunct, first plates of the postero-lateral 

 ambulacra and by the first plates of the anterior zones of the 

 postero-lateral interradia. The sternum is large, is composed of 

 one plate, and is followed by an episternum whicb is composed 

 of two plates placed side by side and crossed by the fasciole. 

 The periproct is longest transversely, and is surrounded by the 

 7-9 plates of the posterior interradium, its membrane is more or 

 less divided transversely and covered with small scales. 



In P. Jeffreysi the first plates of th.e postero-lateral ambulacra 

 are interposed between the first plates of the antero-lateral am- 

 bulacra actinally. Abactinally the last plates of the posterior 

 interradium are separated and intercalated amongst the ab- 

 actinal plates of the postero-lateral interradia. The true mouth- 

 opening is a slit in a membrane without plates, and the direction 

 is vertical. Some large pedicels are on the posterior ambulacra 

 at the peristomial end, but elsewhere all the tentacles or pedicels 

 are pointed and not prehensile ; probably they are all branchial. 

 In P. laguncula the labrum is large and long, but barely separates 

 the first plates of the postero-lateral ambulacra, and this also 

 occurs in P. carinata and P. ceratopyga. The large and numerous 

 spheridia are exposed and are restricted to the lateral ambulacra. 



Grenus Spatagocystis, A. Agassiz, 1879, Proc. Amer. Acad. 

 vol. xiv. p. 206 ; 1881, Beport on ' Challenger ' Ecliini, p. 140. 

 Loven, 1883, Pourtalesia, p. 82. 



Test moderate in size, long, low, ovoid, thin, very slightly 

 incurved anteriorly, and with a short, small and narrow beak 

 posteriorly ; actinally with a downward projecting keel extending 

 from the anterior groove to the beak ; abactinally convex, tumid 

 in front, and lower and narrow behind. 



Apical system disconnected, part excentric in front ; the four 

 basals connected ; three genital perforations, and the madreporite 

 in the right anterior basal. The postero-lateral radial plates 

 separated from the basals by several interradial plates. 



Ambulacra apetalous, uniporous ; plates varying in width in 

 the difi'erent ambulacra, hexagonal. 



