CLARK: AUSTRALIAN AND INDO-PACIFIC ECHINODERMS. 127 



lighter, aud on interbrachial areas more yellowish ; radial shields, deep brown ; 

 arms with about a dozen annulations, approximately two joints wide, of dark 

 purplish-brown. 



In smaller specimens the disc is not distinctly notched at base of arms ; the 

 upper arm-plates are only twice as wide as long ; there are only 7 arm-spiues and 

 the lowest is only equal to 1£ joints ; and (here are only 6-10 dark annulations 

 on the arms. One specimen has the ground color a much darker brown than the 

 others. 



Ponape, Caroline Islands. Rev. A. A. Sturgis coll. Three specimens. The 

 specimens were labelled by Mr. Lyman : " Pectinura septemspinosa ? Probably 

 same as unique original at Leyden. Differs 1. Longer arm-spines. 2. Rather 

 finer granulation. 3. Upper arm-plates not broken." Since Mr. Lyman wrote 

 that label 0. septemspinosa has been found abundantly at Amboina, Ceylon, and 

 Mauritius, and has been fully described and its variations discussed by Brock 

 (1. c), Doderlein (1. c), aud de Loriol (1. c.). In the light of our present 

 knowledge, it seems impossible to consider these specimens from the Caroline 

 Islands as belonging to 0. septemspinosa or any other described species. 



Ophiarachna. 



Miiller and Troschel, 1842. Sys. Ast., p. xiii, 84, 104. 

 Type, 0. incrassata Miiller and Troschel, 1. c. Selected by Liitken, 1869. Add. 

 Hist. Oph., 3, p. 33. 



It is hard to understand why Liitken and Lyman should have considered this 

 genus closely allied to Ophiocoma. If one ignores the length of the arm-spines, 

 it is not easy to point out a single character by which Ophiarachna can be 

 separated from Pectinura. I am unable to see that the position of the spines on 

 the side arm-plate is essentially different even in Ophiocoma from what it is in 

 Pectinura ; it seems to be simply a difference in the size of the spines. Lyman 

 ("Challenger" Oph., p. 173) has pointed out the very important difference be- 

 tween Ophiarachna and Ophiocoma, in the structure of the " peristomial plate," 

 but he fails to note that Ophiarachna strongly resembles Ophiopeza and the other 

 Ophiodermatidae in that feature. Lyman says that in all other particulars the 

 skeleton resembles that of Ophiocoma, but the same is true of the Ophiodermatidae 

 in general, so that it is no argument in support of associating Ophiarachna with 

 Ophicoma. 



As limited by Liitken and in the sense in which the name is now universally 

 used Ophiarachna includes four species. 



Key to the Species of Ophiarachna. 



Radial shields concealed by granulation of disc. 



Arm-spines 4, very rarely 5, on a few basal joints ; colors chiefly 



yellow or greenish incrassata 



Arm-spines 5-7, rarely 4 ; colors chiefly brown of some dark 

 shade. 



