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 calcai'eous plates. In 
A. tonsum we find six plates nearly covering the anal system ; in A. 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. 
microtuberculutwa, covering nearly the whole of the actinostome. The gills 
appear in the youngest specimen 1 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 difierences 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 suuiller number of coronal plates the differences due to growth do not 
seem to be important. 
The sphteridia 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 
ambulacral system. Their number varies from two to four, and sometimes 
as many as six are found in each area. 
The pedicellarije 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 pedicellaria3 are smaller and com- 
paratively more slender than those of A. Jacobi/i, the shaft is very slender, 
and the head quite diminutive ; the sheathed pedicellariae are most numerous 
in the ambulacral area above the ambitus near the abactinal region ; those 
of the interandiulacral areas are slightly larger and far less numerous. 
In a specimen measuring 11 mm. in diameter, there are six interambulacral 
plates; in one measuring 7 mm. there are five. The secondaries and miliaries 
