176 HAWAIIAN AND OTHER PACIFIC ECHINI. 



Ambulacral plates low and numerous, interambulacral 

 only three-fifths — three-fourths as many. 

 Color uniform dark violet ; actinal primary spines 



(except hoofs) equally dark violaceum. 



Color never uniformly dark violet ; actinal primary 

 spines usually lighter than test. 

 Coronal plates, especially abactinally, with wide 

 interspaces of leathery skin. 

 Abactinal interambulacral plates strongly 

 curved or bent adorally so that their ex- 

 panded inner ends are much nearer mouth 

 than the outer ends, in large specimens as 



much as 15 mm. nearer coriaceum. 



Abactinal interambulacral plates nearly or 

 quite straight, though they are not neces- 

 sarily horizontal tessellatum. 



Coronal plates with small interspaces or none. 

 Number of ambulacral plates abactinally lit- 

 tle or not at all exceeding actinal number fenestration. 

 Ambulacral plates abactinally 50-60 per 

 cent more numerous than actinally. 

 Test thin ; valves of large tridentate pedi- 

 cellariaB curved, with blades strongly 

 involute except where they meet at tip gracile. 

 Test rather stout ; valves of large triden- 

 tate pedicellariffi straight or nearly 

 so, the blades not involute but in con- 

 tact for most of their length . . . Owstoni. 



Arseosoma thetidis A. Ag. and Cl. 



Asthenosoma thetidis H. L. Clark, 1909. Bull. M. C. Z., LII, p. 134. 

 Off Botany Bay, New South Wales ; 80 fathoms. 



Plates 66, figs. 6-17; 68-70. 

 Although at first sight this species appears to be closely related to 

 A. Owstoni Mort., careful examination reveals some very decided differences, 

 particularly on the actinal surface. The largest specimen before us meas- 

 ures 180 mm. in diameter; the interambulacra are 63 mm. broad at the 

 ambitus, while the ambulacra are 46 mm. across in the same region. The 

 abactinal system is 32 mm. in diameter, while the actinostome is 41 mm. 

 across. There are 45 coronal plates in each half of an interambulacrum, 

 of which 18 are below the ambitus. The ambulacral plates number 70 from 

 the peristome to the ocular plate ; of these 28 are on the actinal side ; there 

 are 12-14 additional pairs of poriferous plates in each radius of the actino- 

 stome. The primordial interambulacral plate at the peristome is very 

 evident (Pis. 69 ; 70, fig. 2) and bears several small primary tubercles. 



