CLARK: THE ECHINODERMS OF PERU. 327 



Marginal plates more or less concealed not forming a definite 



boundary to a pentagonal body. 



Large species (It up to 150 mm.) with dorsal surface carrying 



numerous big sharp tubercles or spines or both. 



Dorsal surface elevated, with coarse tubercles, granulated at 



base Oreaster occidentalis 



Dorsal surface flat, with numerous, erect, smooth, very stout 



spines Paulia horrida 



Small species (R seldom exceeds 25 mm.) with dorsal surface, 



entirely free from conspicuous spines or tubercles (Asterina). 



Plates of ventral interbrachial areas with only 1 spine 



each A. calcarata 



Plates of ventral interbrachial areas with 2-6 spines 



each A. chilensis 



Disc small or moderate, R more than 3r and usually more than 4r. 



Rays short and thick, about equal to 3r covered, as is the disc 



by groups of crowded blunt spinelets, simulating low paxillae. 



Parasterina obesa 

 Rays longer, 4-10r. 



Disc and rays closely covered with a nearly smooth, granulated 

 coat, without projecting spines, except beside ambulacral 

 furrows. 

 Papular areas arranged in a single (rarely double) broad 



series along each side of ray Phataria unifascialis 



Papular areas arranged in eight series on each ray. 



Madreporic plate very large, its diameter .25 or more of disc 

 diameter ; rays somewhat trigonal ; inner series of adam- 

 bulacral armature made up of approximately equal and 



similar spines Pharia pgramidata 



Madreporic plate much smaller ; rays more or less cylindri- 

 cal ; inner series of adambulacral armature made up of 



alternating large and small spines Ophidiaster ludwigi 



Disc and rays bearing spines or spinelets, sometimes minute and 

 crowded. 

 Spines and spinelets large, unequal and irregularly scattered, 



blunt, covered to tip with scale-like granules . . Mithrodia bradleyi 

 Spines and spinelets bare, often small or even minute. 

 Spinelets very minute, in more or less crowded groups on 



dorsal plates Henricia hyadesi 



Spines well developed, arranged singly on dorsal plates (Echinaster). 

 Dorsal spines rather crowded, in irregular rows . . . E. cribella 



Dorsal spines irregularly scattered E. cylindricus 



Tube-feet in four, more or less distinct series in each ambulacrum. 

 Rays few, never more than eight. 

 Rays 5, with about 11 longitudinal, abactinal series of groups of 



closely crowded granule-like spinelets Stichaster aurantiacus 



Rays normally 6, rarely 5 or 7, with 5 longitudinal series of large, 



widely separated, abactinal spines Asterias gelatinosa 



