92 



SVEN LOVEN, ON POUKTALESIA, A GENUS OF ECHINOIDEA. 



a small siib-pciitangular steriiuiii, coiitii^iious to I 



which is ciitiroly separated froin /, wliile a 2, 



slight re-eiitering- anglc, aiitl a 6 — !), h 5—8 are pcriproctal, 10—16 clorsal, lengtheued, 



a 3 and V h 3, toUowed by 5 a 2, 

 ?, 4 and b 3, 4, 3 form toacthcr a 



sub-hexagonal. 



Hcmipiieustes radiatus Gm. 



Wheri thc successive modifications of 

 the interradium 5 of tbc Spatangida) are 

 followed from the first appearance of the 

 group to our days, it is seen that in 

 the earliest, Collyrites '), after the labrum 

 there begins a double row of alternating 

 plates, in which the plates of the a series 

 are postei'ior to those of the b series, the 

 same order of sequenee that holds good 

 throughout the whole of the Spatangidaj. 

 There is no appearance of a true sternum. 

 In Heniipneustes the ventral plates of ei- 

 ther row, from the b 2 inclusively, extend 

 triangularly aeross the interradium, so as 

 to bring their points within a short di- 

 stance from the opposite ambulacrum, and 



Offaster corculum GouiF. Echinospatagiis Ricordeanus Cotteau. 



to leave only a minute pai"t of a 2 in contact with the labrum, the middle suture 

 thus becoming a zig-zag line of very wide turns. In Holaster ^), Cardiaster, Anancites ), 

 Offaster, this structure is so far modified that the b 2, still more expanded trans- 



1) Études pl. XXIII. 

 -) Ib. pl. XXV. 

 3) Ib. pl. XXIV. 



