92 



SVEN LOVEN, ON POUKTALESIA, A GENUS OF ECIIINOIDEA. 



a small sul)-[)0TitaiigMlar storiiuui, coiitii^niuus to 1 (i 3 and V I) 3, followed by 5 a 2, 

 which is entirely sejjarated from /, wliile <i 2, :>, 4 and b o, 4, o form together a 

 slight re-entering angle, and a 6'— .7, h 5 — H are periproetal, 10 — Hi dorsal, lengthened, 

 sub-hexagonal. 



When the successive modifications of 

 the interradium 5 of the Spatangida} are 

 followed from the first appearance of the 

 group to our days, it is seen that in 

 the earliest, Collyrites '), after the labruni 

 thei'e begins a double row of alternating 

 plates, in which the plates of the a series 

 are posterior to those of the h series, the 

 . same order of sequence that holds good 

 throughout the whole of the Spatangidaj. 

 There is no apjjcarance of a true sternum. 

 In Hemi|)neustes the ventral plates of ei- 

 ther row, from the h 2 inclusively, extend 

 triangularly across the interradium, so as 

 to bring their points within a short di- 

 stance from the opposite ambulacrum, and 



^^Sfc 



Offaster corculum Goi.iu'. Eiliiiiospatagus RicorJcaiius CoTTEAU. 



to leave only a minute ])art of a 2 in contact witli the labrum, tlie middle suture 



of very wide; Xwvw^ 



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

 Offaster, this structure is so far modilied that the h 2. still more expanded trans- 



') Etudes pi. XXIII. 

 -) lb. pi. XXV. 

 ■') lb. pi. XXIV. 



