424 



THE VOYAGE OF H.M.S. CHALLENGER. 



Chorology of the Genus Cnemidaster. 

 a. Geographical distribution : — 



Eastern Archipelago : One species between the parallels of 0" and 10" S. 



Cnemidaster wyvillii, in the Arafura Sea, north-west of the 

 Arrou Islands. 



yS. Bathymetrical range: 800 fathoms. 



7. Nature of the Sea-bottom : Green mud. 



Chorological Synopsis of the Species. 



1. Cnemidaster wyvillii, n. sp. (PL LXVII. figs. 3 and 4 ; PL LXVIII. figs. 3 and 4). 



Rays five. R = 22 mm., r=4 - 25 mm. R > 5 r. Breadth of a ray near the base, 

 3 5 mm. 



Rays eloDgate, cylindrical, slightly flattened actinally, tapering from the base to the 

 extremity. Interbrachial arcs rounded. Disk convex abactinally. 



The disk, which is higher than the rays and regularly convex, is occupied entirely by 

 the primary apical plates, each slightly convex or tumid, which are arranged in the follow- 

 ing manner. The centre is occupied by the dorso-central plate, which is large, almost 

 subconically convex, pentagonal in outline — the augles being more or less rounded, and the 

 right posterior side somewhat encroached upon for the anal aperture. Contingent on the 

 dorso-central is a complete cycle of five basal plates, rather irregular in shape, apparently 

 quadrangular in form, but probably more or less hexagonal, which are smaller than the 

 dorso-central. External to these are the five primary radial plates, irregularly subhexa- 

 gonal in form, larger than the basal plates, and either equal in size to the dorso-central or 

 slightly less ; they are inserted far down between the basal plates, but do not reach the 

 dorso-central. The second radial plate is smaller than the first, hexagonal in form, and 

 broader than long. In the third the length and breadth are equal. In the. radial plates 

 beyond the third and up to the middle of the ray the length is greater than the breadth, 

 but on the outer half of the ray the breadth is greater than the length. Isolated single 

 papula occur at the angles of the plates, but do not extend much beyond the inner half of 

 the ray. The plates of the series on each side of the median radial series are rather 

 smaller, and have the breadth greater than the length on the inner part of the ray, but 



