6 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



d> Abactinal skeleton of rays consisting of independent, compressed arches or costae, as in 

 Brisinga, but the integument between the arches is crowded with numerous spine- 

 less, immersed, fenestrated, thin plates which touch or slightly overlap, and leave 

 the integument entirely flexible; gonads two or four to each ray. 

 «i. First pair of adambulacral plates united, and joined to their upper side is a united 

 pair of marginal plates, four in all, as in Brisinga; proximal adambulacral plates 

 higher than long; gonads two to each ray. Genotype Brisinga (Stegnobrisinga) 



placoderma Fisher Stegnobrisinga Fisher. 



e 2 . First pair of adambulacral plates not joined together by their lateral faces, but separate; 

 first pair of marginal plates is not united by their lateral faces but only by the 

 adoral ends, to which also is closely united the lower end of the interradial plate, 

 forming a rude inverted Y, of which the angle is the apex of the interbrachial angle 

 and the arms are the first marginal plates; proximal adambulacral plates not 

 higher than long; gonads two or four to each ray. Genotype, Brisinga panamensis 



Ludwig Astrolirus Fisher. 



cP. Abactinal skeleton of genital region of ray composed of a uniform armor of thin, spin- 



iferous, more or less overlapping plates, but not of independent arches or costae. 



e 1 . Primary plates of disk much larger than the others which are small; gonads two to 



each ray; an interradial plate, vertical in position, is interpolated between the 



proximal ends of the first adambulacral plates, and touches the mouth plate, but 



does not encroach upon the actinal surface as in Colpasler. Genotype Belgicella 



racovitzana Ludwig Belgicella Ludwig. 



e 2 . Primary plates of disk not conspicuous, and not distinguishable from the other abac- 

 tinal plates of disk. 

 p. An azygous interradial plate, shield-shaped and conspicuous superficially on the 

 actinal surface, separates the first pair of free adambulacral plates; gonads 



unknown. Genotype, Colpasler sculigerulus Sladen Colpasler Sladen. 



p. First pair of adambulacral plates not separated by an azygous plate but united as in 

 Brisinga, and above them is a pair of conspicuous, united, first marginal plates — 

 four in all in each interradial angle; gonads numerous, in a series along either 



side of the ray. Genotype Freyella fecunda Fisher Freyellaster Fisher. 



c 2 . No syzygy, or nonmuscular symphysis, between the first and second adambulacral plates 

 nor between the upper part of the second and third ambulacral plates, but a normal 

 muscular symphysis not different from the others; no marginal plates directly above 

 the first pair of adambulacral plates; the latter are not united except sometimes by 

 the proximal ends; touching the lower end of the interradial plate is a pair of very inconspic- 

 uous plates lying in the same plane, and superficially appearing to be a part of the inter- 

 radial plate.* These are really the outer ends of the mouth plates which project dorsally 

 behind the first pair of adambulacral plates. The latter, by sometimes touching at 

 their inner ends or apposing their lateral faces, segregate this dorsal portion of the mouth 

 plates from the actinal, spine-bearing part. For this reason they may easily be mis- 

 taken for two entirely independent plates. 

 d 1 . Abactinal skeleton of ray composed of a uniform armor of thin, spiniferous, more or less 

 overlapping plates, as in Freyellaster, not of spaced independent arches or costae; 

 furrow spinelets not modified or expanded at the tip. Genotype, Freyella spinosa 



Perrier Freyella Perrier. 



d 2 . Abactinal skeleton of ray composed of independent transverse arches, composed, on radial 

 area, of flattened overlapping plates (not of elongate, narrow, more or less com- 

 pressed ossicles as in Brisinga) ; these costae bear spinelets and are separated by inter- 

 vals devoid of plates; furrow spinlets with curiously modified expanded tips. 

 Genotype, Aslrocles aclinodetus Fisher Astrocles Fisher. 



» The interradial plate described by Ludwig in Belgicella is very probably the true interradial plate plus these extreme outer 

 ends of the mouth plates, which, unless treated with potash, appear to be a part ot the interradial plate. (Ludwig, 1903, p. 60.) 



