326 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



and r stand tor the minor radius, i. e. from the center of the mouth to the middle 

 of an interbrachial margin ; in a perfectly circular starfish, we would say, R = r ; 

 in an ordinary pentagonal starfish R = r +, while in a starfish with long rays 

 R = 6r, more or less. 



Key to the Starfishes of the Peruvian Coast. 



Upper surface covered with paxillae; general form decidedly flattened; 

 inferomarginal plates with long spines forming a conspicuous lateral 

 fringe along the ray. 



Superomarginal plates conspicuous in contrast to paxillae (Astropecten) . 

 Superomarginal plates, each with one or two prominent vertical 



spinelets. 



Superomarginal plates at base of ray (but not in interbrachial area) 

 with two spinelets each, one on inner edge and one near outer 



margin A. erinaceus 



Superomarginal plates with only a single spinelet each, except that 



a few near tip of ray may carry a second small one . . A. peruvianus 

 Superomarginal plates without spinelets ; some of those near base of 



ray may carry, each, a small rounded tubercle A.fragilis 



Superomarginal plates reduced and not readily distinguishable be- 

 neath paxillae (Luidia). 

 Each paxilla in third longitudinal series at side of ray carries, except 



near tip of ray, a long slender spine L. phragma 



None of the paxillae carry long and slender spines. 



Inferomarginal plates, each with a longitudinal (transverse to long 

 axis of ray) series of 4-6 flattened, usually blunt or truncate 

 spines; many paxillae have central spinelet enlarged into a blunt 



elevated tubercle or sharp, stout spine L. bellonae 



Inferomarginal plates, each with 2 or 3 long, sharp spines; no 

 paxillae (except occasionally some along sides of ray) with large 



central tubercle or spinelet L. Columbia 



Upper surface without paxillae; sides of ray without lateral fringe of 



slender inferomarginal spines. 



Tube-feet in two series in each ambulacrum ; rays normally five. 

 Disc large and rays short; form often pentagonal; R never more 



than 2.5r. 

 Marginal plates conspicuous, forming a definite boundary to the 



more or less pentagonal body. 

 Marginal plates and disc free from large~spines. 



Superomarginal plates 19-20 on each side of each ray (in speci- 

 men 75 mm. across), covered by a close pavement of crowded 



granules Tosia verrucosa 



Superomarginal plates 15-16 on side of ray (in specimen 75 mm. 

 across), covered by a coat of distinct granules coarser than 



those of disc Odontaster singularis 



Some marginal or disc plates or both, carry large spines Nidorellia armata 



r 



