352 ’ DHE ATLANTIC. [cHap. v. 
nal marginal plates on each arm are fused together, and bear 
two diverging spines, one on either side, and above these on 
the dorsal aspect.a central spine set on a low tubercle. In the 
re-entering angles between the arms the two central pairs of 
marginal plates are closely covered with minute flattened scales 
Fie. 97.—Porcellanaster ceruleus, WyviLLE THomson. Oral surface. Natural size. (No. 45.) 
inserted on edge, and arranged in vertical rows. This is a most 
characteristic style of ornament; it looks as if there were a 
little brush between each pair of arms. The perisom of the 
dorsal surface is loaded with narrow calcareous plates which 
run together toward the ends of the arms so as to form an 
almost continuous calcareous investment; paxillee are scattered 
over the disk, and the outer layer of the perisom of the disk 
has a very delicate color, ranging from a pale to a tolerably 
strong cobalt-blue. The calcareous plates are clear white, with 
somewhat of a porcellanous lustre, and look harder than the 
surface plates usually do in star-fishes. The madreporic tuber- 
cle is large, with sub-parallel grooves and ridges; the excretory 
opening is very distinct in the centre of the dorsal perisom of 
