130 HAWAIIAN AND OTHER PACIFIC ECHINI. 



LiSSODIADEMA. 



Mortensen, 1903. Rev. Suisse de ZooL, XI, p. 393. 

 Type-species, Lissodiadema Lorioli Mortensen, 1903. Rev. Suisse de ZooL, XI, p, 393. 



The two specimens upon which this genus and species are based were 

 taken at Amboina. They measure 10 and 22 mm. in diameter and were 

 regarded by de Loriol, who first described them, as young individuals, pos- 

 sibly of Asthenosoma. Mortensen has shown that they can hardly be young 

 Echinothurids, and are almost certainly Diadematids, though they are unlike 

 any known young of the latter family. The genital pores are undeveloped, 

 and they appear in other ways to be immature, yet it seems to be necessary 

 to give them a genus of their own, at least for the present. 



Leptodiadema. 



A. Agassiz and Clark, 1907. Bull. M. C. Z., L, p. 238. 



Type-species, Leptodiadema purpureum A. Agassiz and Clark, 1907. Bull. M. C. Z., L, 



p. 239. 



This genus is established for a very small Diadematoid apparently quite 

 different from any known genus. The size, form, and spines remind one of 

 Lissodiadema, and the abactinal system is not altogether unlike that genus, 

 but the tuberculation is entirely different. The test is flattened both actinally 

 and abactinally ; the ambulacra narrow, with pores in a single straight series 

 not becoming crowded at the actinostome. Each ambulacrum carries a douljle 

 series of primary tubercles extending from the abactinal system to the ac- 

 tinostome. The coronal plates are numerous, each with a large primary 

 tubercle at the outer end. Below the ambitus these tubercles are increas- 

 ingly nearer the center of the plate, so that the two series converge and 

 meet at the actinostome. Beginning at the fifth from the abactinal system, 

 each coronal plate carries a second somewhat smaller tubercle at the inner 

 end, and these two series terminate at about the fourth plate from tlie acti- 

 nostome. Secondary spines few, miliaries almost wanting. The primary 

 tubercles are low, indistinctly perforate and apparently finely crenulate. 

 From the ambitus to the abactinal system the ambulacral tubercles are small. 

 On the actinal side the tubercles of both systems are nearly uniform in size. 



Abactinal system moderate, the oculars on each side of the madreporic 

 plate and the left anterior one are excluded from, while the other oculars 

 extend to, the large anal system, which is covered witli two rows of platca, 



