198 HAWAIIAN AND OTHEE FACIFIC ECHINI. 



those by Doderlein of S. hiseriatmn and those here given of S. obscurum 

 (PI. 89). 



The actinal system of S. Grimaldii as figured by Kcehler shows its 

 plates to be closely packed with small secondaries arranged horizontally 

 completely connecting the sutures ; nor does Koehler figure any actino- 

 stomal ambulacral pores. The actinostomal ambulacral plates of S. 

 giganteum (PI. 85, fig. 2) are, on the contrary, well separated, arranged in 

 ten vertical series. 



In all the other species of Sperosoma, of which the abactinal system has 

 been figured, the genital and ocular plates are most distinct, and while each 

 genital is made up of many plates, it is a simple matter to distinguish them 

 from the anal plates. However, such is not the case in S. giganteum, for it 

 seems impossible in the maze of polygonal anal plates, with their close 

 granulation (PI. 85, fig. 1) encroaching upon the broken genital and ocular 

 plates, to distinguish the limits of the latter. It is of course possible that 

 this great breaking up of the plates of the abactinal system may be due to 

 age. The madreporic plate is irregularly circular, surrounded with small 

 plates and edged with miliaries. It is the only genital one can trace with 

 any certainty (PL 85, fig. 1), and of the oculars, the left posterior is the only 

 one at all distinct. 



The pedicellarioe are interesting, for in addition to tridentate pedicel- 

 larise similar to those of S. biseriatum Dod. (but seldom with valves as 

 much as two millimeters long) we find ophicephalous and triphyllous 

 pedicellariae abundant. The latter are not peculiar, but the former are 

 almost exactly like those figured by Mortensen (1903, PI. 14, fig. 23) as 

 characteristic of his proposed new genus " Tromikosoma " ! In no other 

 respect, however, does this species resemble that group. All the pedi- 

 cellariae are numerous but small. The tridentate and triphyllous occur 

 practically everywhere, but the ophicephalous seem to be confined to the 

 ambital region. 



The tridentate (PI. 64, fig. 9) are provided with comparatively short stalks 

 and have a very short neck ; the stalks slightly exceed the head in large 

 examples, but are three or four times the head in small ones. The valves 

 (PI. 64, figs. 10-11) range from .40 to 1.20 mm., but are most commonly less 

 than a millimeter. They are blunt, often decidedly rounded at tip, and the 

 margins are very slightly sinuate, or a little concave at the base of the blade. 

 In large examples there is more or less of a calcareous mesh-work in the blade. 



