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 paxillse 

 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 PI. 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 paxillse are scarcely shown with sufficient 

 emphasis, though the dark areas well represent their presence. 



3. Hyphalaster inermis, Sladen (PI. XXV. figs. 4-6 ; PI. XXVIII. figs. 5-8). 

 Hyphalaster inermis, Sladen, 1883, Journ. Linn. Soc. Lond. (Zool.), vol. xvii. p. 239. 



Pays five. P= 20 mm. ; r = 8'5 mm. R < 2*5 r. 



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 arc emphasises the 

 marked pentagonal contour of the body-disk. 



The abactinal area is covered with closely crowded paxillae, the whole disk as well as 

 the base of the rays being uniformly packed. The paxillse 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 Ctcnodiscus. 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. 



