52 BULLETIN 76, UNITED STATES NATIONAL MUSEUM 



The young of M. platyacanthum of comparable size have much thicker rays, 

 shorter spines, numerous abactinal miliary spinelets, broader furrows, and four- 

 ranked tube-feet. 



MYXODERMA PLATYACANTHUM (H. L. Clark) 



Plate 15, Figure 3; Plate 16, Figures 2, 2a; Plate 23, Figure 2; Plate 24, Figure 1; Plate 25, Figures 1, 2 



Zoroaster platyacanlhus H. L. Clark, Bull. Amer. Mus. Nat. Hist., vol. 32, art. 8, July 9, 



1913, p. 199, pi. 44, figs. 1 and 2. 

 Myzoderma platyacanthum Fisher, Ann. and Mag. Nat. Hist., ser. 9, vol. 3, 1919, p. 392.— 

 Clark, Mem. Mus. Comp. Zool., vol. 39, No. 3, 1920, p. 99; Bull. Amer. Mus. Nat. 

 Hist., vol. 48, art. 6, 1923, p. 152. 

 Diagnosis— Rays five. R 96 mm.; r 10.5 mm.; R = 9 r; breadth of ray at 

 base, 12 mm. (Specimen from station 4321; the type measures R 67 mm.; r 9.5 

 mm.; r = 7 r.) Disk small, usually flat-topped or slightly sunken in middle; rays 

 convex, without a carinal ridge, so that the dorsum has a somewhat flattened appear- 

 ance; rays slender, nearly parallel-sided to near the end which is not attenuate; 

 abactinal spines and spinelets sharp, rough, bristling, rather short; actinolateral 

 spines bristling, broad, and flat; especially characterized by open skeleton, large 

 papular areas, small adradial plates, flattened actinolateral spines, an adradial series 

 of primary spines, broad furrows, four-ranked tube-feet throughout, and super- 

 ambulacral plates. 



Description. — Primary plates of disk not especially conspicuous, stellate, the 

 skeleton coarse with relatively large meshes containing one or two large papulae 

 which do not occupy all the area. Spines, spinelets, and pedicellariae as on rays. 

 Dorsal surface of ray not carinate, nor are the median radial series of spines 

 different from the others. Carinal plates with four or six lobes, and only a little 

 larger than the four-lobed marginal plates. There are three series of four-lobed 

 actinolateral plates, the lowermost plates being much longer than wide owing to a 

 shortening of the transverse processes. Between the carinal and superomarginal 

 plates is a series of mostly three-lobed (sometimes only two-lobed) adradial plates, 

 which are very inconspicuous in all but the largest specimens (where they bear pri- 

 mary spines) owing to encroachment of the adjacent series. These plates extend to 

 the end of the ray but distally are quite small, rather irregular, and pretty completely 

 hidden by the lobes of neighboring plates. The whole skeleton is very open-meshed 

 for a member of this family, and resembles that of a Pedicellaster. (For details of 

 skeleton, see pi. 16, fig. 2a.) 



There are two series of papular areas between the carinal and superomarginal 

 plates, and four lateral series, each with a large, swollen papula (or rarely two), which 

 does not occupy the whole area. 



In fully grown specimens each carinal, marginal, actinolateral, and a variable 

 number of adradial plates carries a central stout, tough spine. Those on the carinal, 

 adradial, and marginal plates are conical, usually sharp, and about 2 mm. long in the 

 largest specimen. The spines of the upper actinolateral series are a little longer than 

 the inferomarginal, usually slightly flattened toward the tip, which is broader and 

 blunter than that of the marginal spines. The second row has the longest spines 

 (3 to 4 mm.). These are much flattened and widened, truncate, or lanceolate-sub- 

 acute, often broadly grooved, and incipiently bifid in extreme cases. The lowest 



