8 SVEN LOVEN, ON A RECENT FORM OF THE ECHINOCONID^.. 



pcarance of resting with tlieir bases on the iutcrradia, in this 

 also like the auricles of the recent form. 



About the genus Anorthopygns we are still ignorant in 

 this respect. 



Thns Pygaster, Pilens, Holectypus, and our recent form, 

 ao-ree in havino- the auricles of every ambulacrum directed 

 longitudinally Avith relation to the ambulacrum, and separate. 

 In Discoidea, and I believe in Galerites, their strueture is an- 

 other. When in a specimen of the Discoidea cylindrica Agäs- 

 siz the interiör has been carefully cleared of the chalk, the peri- 

 stome, icoodcut f. 7, is seen to be formed of the five narrow, 

 projecting ambulacra, alternating with the five broad, less pro- 

 minent interradia. Wherc approaching to the peristome the 

 ambulacra have their poriferous zones approximating and the 

 middle part of their first pairs of plates swelling inwardly and 

 protuberåint, with the single pores arrauged all around the pe- 

 ristome according to the general formula I a, II a, III h, IV «, 

 V h, and I h, II h, III a, IV h, V a. At the same time their 

 lateral parts are seen to expand and to support on each side a 

 slightly reclining auricle, thin at its upper free margin and 

 therc at its ambulacral angle presenting a triangulär part pro- 

 jecting över the zone and made distinct from the auricle itself 

 by an impressed line visible in all on the adoral surface, 

 7coodcut f. 1, 2. The succession of elevated ambulacral plates 

 by which the auricle is supported behind, /. 2, is very short, 

 /. 3, and in no wise forms a ridge prolonged into the ambula- 

 crum, but simply a transverse swelling. The auricles thus con- 

 structed do not stånd separate and free as in Pygaster. On 

 the outward side, that is on the margin facing the interradium, 

 each of them is imited to that of the ncxt ambulacrum by a 

 wall restino- on the interradium, nearly attainino- the height 

 of the auricles, almost rectilinear at its upper margin, and 

 slightly hollow on the adoral side. A median vertical suture 

 is to be folioved downwards. At the basc are clearly seen 

 the odd median first plate ^), and on each side of it the peri- 

 stomal beginnings of the two second plates, which are of 

 conspicuously unequal size. The ridges, which in each inter- 

 radium radiate towards the periphery, gradually becoming 

 thinner and higher so as to divide the ventral part of the 



') See Etudes s. 1. Echinoidées, Pl. XIV, fig. 125. 



