138 NATUJRAL HISTORY BULLETIN 



or more smaller ones at its base. The sutures are more or less 

 fasciolated. 



Interactinal plates are present, in small numbers. They are 

 spinulose and may have a central larger spine, and often sutural 

 pectinate pedicellariae. 



The adambulacral plates have a prominent inner angle, which 

 bears, at its edge, a row of six to ten or more spines ; the actinal 

 side has one or two large central spines and often a smaller ac- 

 cessory one, besides some small basal spinules. Sometimes there 

 are two or more equal spines. 



The pericellariae are fasciculate or pectinate. They may occur 

 on any series of plates; rarely on the adambulacrals. Those on 

 the interactinals are pectinate and usually largest. They may 

 cover the suture between two plates, or occupy a single plate. 

 Those on the dorsal plates are small, fasciculate, and are not al- 

 ways present. 



The dorsal muscle-bands are well developed. Their proximal 

 ends are attached to one or two ambulacral plates and to the 

 superomarginals. 



This genus is closely allied "^o Pontaster. The latter (typical) 

 differs chiefly in having two-valved pedicellarige, instead of pec- 

 tinate ones; and in their position, for they mostly occur on the 

 adambulacral plates. These plates also have two or more larger 

 spines on the actinal side, instead of one or two, as is usually the 

 case in this genus. 



The number of actinal adambulacral spines cannot be trusted 

 as a true generic character, for it varies on different plates of the 

 same ray, but in this genus they seldom ^orm a cluster, as they 

 sometimes do in Pontaster. 



Typical Pontaster also has more crowded papulae and more 

 specialized papularia, with a more complex calcified inner struc- 

 ture, and they are therefore more swollen externally than in thi? 

 genus, in which they are sometimes scarcely at all swollen, espe- 

 cially in the young. 



Fisher (1911&) made the bivalve form of the pedicellarige and 

 their position on the adambulacral plates the most diagnostic 

 character to separate Pontaster from Pectinaster. This seems 

 to me the most useful and most available method, except in those 

 rather unusual cases when no pedicellariae occur, and in the very- 

 young. 



