a | 
Nia —<—" ¢ | 
id 
3800 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
similar plates in the left row of the ambulacrum; those 
on the right were similarly apposed to one of the plates of 
the adjacent interambulacrum. It follows from this 
arrangement that the suture uniting the two rows of 
plates of an ambulacrum forms a slightly irregular zigzag 
line. Examination of such an ambulacral plate with a 
lens reveals two oblique sutures (swt.), which sub-divide 
it into three minor plates. Hach of these is traversed by a 
pair of pores (po.), through which the tube feet communi- 
cate with their ampulle (Pl. IV., fig. 29), hence they are 
called pore plates. 
The great majority of the ambulacral plates bear one 
large tubercle, and a variable number of smaller ones, 
upon which spines and pedicellariz are seated. The apical 
extremity of each ambulacrum is composed of two small 
pore plates, which usually meet in the middle line. As 
growth of the test, as a whole, proceeds, such primitive 
plates are added between those already formed and the 
radial plates of the apical system; and it is by the ultimate 
fusion of three such primitive pore plates that the ordin- 
ary ambulacral plates are formed. As new pore plates 
appear, their predecessors are pushed towards the oral 
ends of the ambulacra; and, as they grow unequally, 
some remain in contact at the median suture, and are 
called primary plates. Others are pushed aside from the 
suture, and are called secondary plates. The outer ends 
of all the primary and secondary plates, except those last 
formed at the apices of the ambulacra, are in contact 
with the plates of the adjacent interambulacra. 
The plates of the interambulacra (Pl. I., fig. 4) are 
similar in shape to those of the ambulacra, but are much 
larger, and are not sub-divided by suture. Hach of the 
four or five last formed in each row bears one primary 
tubercle, and a variable number of very small ones. ‘The 
