126 HAWAIIAN AND OTHER PACIFIC ECHINI. 



from the exterior the sutures of the primary plates both of the ambulacral and 

 interambulacral areas in spite of the maze of granules which cover the actinal 

 surface (see PI. 56, figs. 1, 5). These sutures, Mortensen says, can be traced 

 in gramilatwn only by examining the interior. 



On the inner surface of the test the lapping of the interambulacral plates 

 on the abactinal side is plainly seen ; it is more marked along the sutures of 

 the median line than along the horizontal sutures. The interambulacral. 

 plates carry minute spicular granules irregularly scattered along the median 

 and lateral side of each plate. These granules become more numerous to- 

 wards the ambitus. At the ambitus, where the rows of large tubercles begin, ' 

 their presence is indicated by deep circular cavities which extend both in 

 the interambulacral and ambulacral areas nearly to the actinostome. 



Fig. c. Fig. d. 



The splitting of the interambulacral plates which was noticed in Astro- 

 pyga (A. Agassiz, Challenger Echini PI. 10% fig. 9) is found mainly in the 

 larger actinal plates near the ambitus ; in Chaetodiadema the splitting of the 

 interambulacral plates takes place, according to Mortensen, at the median 

 end. See figs, c and d. 



Seen from the interior the ambulacral and interambulacral plates of 

 the actinal side are covered with rows of deep pits corresponding to the rows 

 of primary and secondary tubercles, and the surface of the plates is 

 covered with irregularly arranged patches of minute granular spicules. Tlie 

 aml)ulacial plates on the actinal face show a splitting up into irregular 

 plates like tliat of the interambulacra. The lajiping of tlic plates on the 

 actinal surface is not as clearly indicated as on the abactinal. The lapping 

 of the intcrauibidaeral sutures on the actinal surface, when compared to that 

 of the abactinal side, is most irregular, owing to the crowding of the tubercles. 



