138 ] 
RECORDS OF W.A. MUSEUM. 
circular plates, in three-five longitudinal series, and the much larger 
terminal superomarginal plates. In diibeni when R=4i, r=only 17; 
the arm is 20 mm. broad at base and only 11 mm. at a point half- 
way between tip and centre of disk ; there are 50-60 superomar- 
ginals. In stibarus with R=qi mm., r= 23 , br=26 and br at the 
middle of arm=20 mm. there are only 40 superomarginals. In 
dtibeni , theabactinal plates on the rays are in only 1-3 series, swollen 
and much elongated parallel to the axis of the ray, but in stibarus, 
they are nearly or quite flat and more or less circular. From 
pulchellus, the type of the genus, stibarus differs greatly in the much 
closer plating of both surfaces, with smaller granules between the 
plates, and in the pedicellariae being abactinal instead of actinal. 
If the abactinal tubercles are a constant feature of adult stibarus, 
they will serve as an additional specific character. 
Fisher 1 has stated as a character of Pentagonaster that it is 
the last plate of each marginal series which is largest and not the 
penultimate or antepenultimate. Examination of four specimens of 
pulchellus and three of diibeni shows that while it is the last of the 
superomarginals which is largest, of the inferomarginals, the last is 
smallest and the penultimate is largest, though sometimes 
approximately equalled by the antepenultimate. In the type of 
stibarus as already stated, the antepenultimate superomarginal is 
largest, but in the smaller specimen in only two of the ten possible 
cases is the antepenultimate ‘largest, in the other eight it is the 
penultimate which exceeds. This difference, however, is more im- 
portant on paper than in reality for in every series in the two 
specimens it is the fourth plate from the median interradial line 
which is enlarged and the apparent difference is due to whether one 
or two small plates are developed distal to it. The characteristic 
arrangement of the marginals in Pentagonaster may then be stated 
thus: The last superomarginal plate is enlarged and swollen and 
overlies the last two or three inferomarginals ; of these the penulti- 
mate, or sometimes the antepenultimate, is largest. 
1 
1911. 
Bull. U S. Nat. Mus., No. 76, pp. 166 and 171. 
