Genera and Species of Brlsingida?. 



lower end of the interradial plate 

 is a pair of very inconspicuous plates 

 lyinu- in the same plane, and super- 

 ficially appeariu<>- to be a part of the 

 interradial plate *. These are really 

 the outer ends of the mouth-plates 

 which project dorsally behiud the 

 first pair of adambulacral plates. 

 The latter, by sometimes touching 

 at their inner ends, segregate tins 

 dorsal portion of the mouth-plates 

 from the actinal spine-bearing part. 

 For this reason they may easily be 

 mistaken for two entirely inde- 

 pendent plates (see fig. 7). 



Fig-. 7. 



425 



Freyellidea microplax. Lateral view of interbrachium ; the surface of 

 the interradial and mouth-plates is dotted. 



e^. Abactinal skeleton of ray com- 

 posed of a uniform armour of 

 thin, spiniferous, more or less 

 overlapping plates as in Freyella, 

 not of spaced, independent arches 

 or costse ; furrow spinelets not 

 modified or expanded at the tip. 

 Genotype, Freyellidea onici-oplax, 

 Fisher Freyellidea, Fisher, 



e^. Abactinal skeleton of ray con- 

 sisting of independent transverse 

 arches, composed, on radial area, 



* The interradial plate described by Ludwig in Belgicella is perhaps 

 the true interradial plate plus these extreme outer ends of the mouth- 

 plates, which, unless treated with potash, appear to be a part of the 

 interradial plate (Ludwig * Belgica ' Keport, 1903, p. 60). 



