26 ASPIDODIADEMA ANTILLARUM. 



size; the former carry larger miliaries than the latter; the ocular plates are 

 somewhat pentagonal, while the genital plates appear more rectangular with 

 rounded corners. 



The anal system is covered with minute irregularly arranged plates; those 

 immediately surrounding the anal system are larger, and in older specimens 

 soldered together to form an irregular calcareous ring round the opening. 

 This ring carries larger miliaries than the other anal plates. The anal tube 

 itself extends as a sort of proboscis beyond the general level of the anal 

 system. It is strengthened by delicate longitudinal calcareous plates. In 

 A. ionsum we find six plates nearly covering the anal system ; in .1. microtuber- 

 culatum there are a number of smaller plates arranged irregularly round the 

 anal opening. 



The actinal system is strengthened by ten large buccal plates, as in A. 

 microtuberculatum, covering nearly the whole of the actinostome. The gills 

 appear in the youngest specimen I have examined (3.5 mm. in diameter) as 

 one-forked digits; they do not even in larger specimens take a great develop- 

 ment, judging from alcoholic specimens. 



We do not find in younger specimens any marked differences from the 

 adult in the structure either of the actinal or of the abactinal system. The 

 primary spines are proportionally larger and longer, but with the exception 

 of the smaller number of coronal plates the differences due to growth do not 

 seem to be important. 



The sphseridia of this species when fully grown are large, globular, short- 

 stemmed ; they are placed mainly in the abactinal region of the test, but are 

 also found near the actinostome and scattered over the whole length of the 

 ambulacra! system. Their number varies from two to four, and sometimes 

 as many as six are found in each area. 



The pedicellariae are similar to those previously described as characteristic 

 of the genus; they are either long narrow-headed and long-stemmed, or 

 short-headed and stout-stemmed, or short-stemmed and pyramidally headed; 

 the last are not numerous. The sheathed pedicellariae are smaller and com- 

 paratively more slender than those of A. Jacobyi, the shaft is very slender, 

 and the head quite diminutive; the sheathed pedicellariae are most numerous 

 in the ambulacra! area above the ambitus near the abactinal region; those 

 of the interambulacral areas are slightly larger and far less numerous. 



In a specimen measuring 1 1 nun. in diameter, there are six interambulacral 

 plates; in one measuring 7 nun. there are five. The secondaries and miliaries 



