GYMNECHINUS MEGALOPLAX. 287 



The five species of this genus occur in the Indian region ranging from Mauri- 

 tius and the Persian Gulf to Australia, Siam, and Macclesfield Bank. They are 

 all of small size, rarely exceeding 25 mm. in diameter. They may be distin- 







guished from each other as follows: 



Spines white; auricles meet and form an arch. 



Suranal small, without a tubercle, in contact with only two genitals; gill-slits 



shallow Robillardi. 



Suranal large (much larger than any one of the oculars) in broad contact with four 



genitals and, like each of them, carrying a tubercle; gill-slits deep megaloplax, 



Spines more or less colored; auricles not united. 



Interambulacral plates at ambitus with the tubercles arranged around the primary 



or scattered, not arranged in definite horizontal series. 

 Primary spines rose-purple or reddish, with light tips, not banded .... pulchellus. 



Primary spines with 1-3 bands of red versicolor. 



Interambulacral plates at ambitus, each with at least four large secondaries which 

 form a horizontal series with the primary tubercle; sutures between inter- 

 ambulacral plates, especially the horizontal ones, well marked; primary spines 

 violet or pale reddish with a broad indistinct violet band, light tipped epislichus. 



Gymnechinus megaloplax, 1 sp. nov. 

 Plate 102, figs. 2, 3. 



The single specimen at hand is 17 mm. in diameter and 8 mm. high. The 

 test is thus somewhat flattened and the actinostome is slightly sunken. The 

 abactinal system measures 6 mm. in diameter along the axis IV-1 but only a 

 trifle over 5 mm. along the axis V-2. The actinostome is symmetrical and is 

 about 7 mm. across. The longest primaries are 4 or 5 mm. in length and occur 

 at and below the ambitus. There are 15 interambulacral and 19 ambulacral 

 plates in each column. The ambulacra are very wide, only a little narrower 

 at the .ambitus than the interambulacra. There is a well-developed primary 

 tubercle on every coronal plate, those of the ambulacra being slightly smaller 

 than those of the interambulacra. The secondary tubercles are small and 

 scattered; only at the ambitus, in the interambulacra, do they become notice- 

 able; there, there is one on each end of the plate, which is much larger than the 

 others, but not nearly as large as the primary. The genital plates (PL 102, fig. 3) 

 vary much in size, 3 is the largest, 2 and 4 are a little smaller and of about equal 

 size, 5 is distinctly smaller, while 1 is much the smallest, being no larger than 

 ocular I. Each of the genitals, except 1, carries a single conspicuous tubercle. 

 The oculars are of about equal size and each of them carries a small secondary 



1 fJif\o.<i (fif\aX) = big + 7r\d = a plate. 



