162 THE VOYAGE OF H.M.S. CHALLENGER. 
species; and I do not think it necessary to make an additional plate for the illustration 
of the larger specimen. The differences appear to me to be clearly due to age, and 
may be summarised briefly as follows :— 
The smaller specimen, which measures R=14 mm., r=7'5 mm., is shorter in the 
ray, the minor radial proportion being about 53°5 per cent., whilst in the larger example 
it is 41°6 per cent. The rays have also a more arched appearance abactinally. The disk 
in the smaller example is more inflated, and this causes the specimen to appear deeper 
in the lateral view. In the larger specimen, which is described, the larger paxille 
have a greater number of spinelets and are more numerous on the disk. The secondary 
row of small granules noticed on the outer margin of the adambulacral plates near the 
mouth in the larger example is not present in the smaller specimen, or only represented 
by mere rudiments on a few of the innermost plates. In like manner the small isolated 
granules noticed on the actinal intermediate plates of the larger specimen are very few 
and far between on the small one. Also the actinal intermediate plates are relatively 
deeper and less broad in the small form. 
In the drawing of the abactinal view on Pl. XXI. fig. 1, the membranous area 
between the marginal plates of the two sides of the ray is rather too broad. In the 
specimen the sides of the ray are somewhat compressed, and the figure is a restoration 
to the supposed normal condition ; but the breadth indicated appears to me certainly too 
great. The very striking groups of larger paxillae are scarcely shown with sufficient 
emphasis, though the dark areas well represent their presence. 
3. Hyphalaster inermis, Sladen (Pl. XXYV. figs. 4-6; Pl. XXVIII. figs. 5-8). 
Hyphalaster inermis, Sladen, 1883, Journ. Linn.’Soc. Lond. (Zool.), vol. xvii. p. 239. 
Rays five. R=20mm.; r=8'5 mm. R< 2‘57, 
Marginal contour stellato-pentagonoid. Rays well developed, slender, round, and 
tapering but slightly. The disk is depressed, not inflated, and both the abactinal and 
actinal surfaces stand on a level with the edges of the marginal plates. The minor 
radius is in the proportion of 42°5 per cent. The interbrachial arcs are very wide 
and expansive, the curve being slightly flattened at the summit of the are emphasises the 
marked pentagonal contour of the body-disk. 
The abactinal area is covered with closely crowded paxille, the whole disk as well as 
the base of the rays being uniformly packed. The paxillee are very fine and small, and 
are made up of about five to ten spinelets. Towards the margin they become smaller, and 
also in the centre, where they are very compact—a slightly prominent peak being formed 
as in Clenodiscus. A slight elevation of the surface is present in the median radial line, 
opposite the base of each ray, and at about one-third of the distance from the margin to 
the centre, 
