214 ECHINODERMATA—ECHINOZOA SUB-BRANCH III 
One of the basal plates, namely, the right anterior one, is generally larger 
than the rest, and serves at the same time as the madreporic body. This 
fact greatly facilitates the orientation, whenever the madreporite is preserved ; 
unfortunately, however, it is indistinguishable in the majority of fossil speci- 
mens. The anterior radial plate is that lying to the front and on the left 
hand of the madreporite-bearing basal; and it surmounts the odd or anterior 
ambulacrum (Fig. 344, J). It is evident from the inspection of any Sea- 

IIL 
Fia. 344. 
Apical systems of A, Palwechinus. C, Salenia. D, Peltustes, enlarged. The [Amb are designated I—V. 
urchin that a plane passing through the odd ambulacrum, mouth, anus, and the 
posterior interambulacrum, will divide the test into two symmetrical halves. 
In the Exocyclica the basal plates may be in contact at their sides, forming 
a compact system (Fig. 345, B, D); or they may be separated by some of the 
radial plates which 
unite along the 
median line and 
push the posterior 
basals backward, 
forming an elongate 
system (Fig. 345, C). 
When the two pos- 
terior ambulacra 
(bivium) do not 
terminate at the 
summit in line 
with the other 
three (trivium), and 
are surmounted by 
radials placed far 
posteriorly, — the 
system is said to 
be disjunct or dis- 
connected, The pos- 
Fia. 345, terior radials are 
Gris 1D, MEG Le Cn hep oarie B, Holectypus. C, Hybo- then separated 
a: from the postero- 
lateral basals by a number of interambulacral plates intercalated along the 
dorsum (Fig. 345, 4). 
In the Clypeastridae, Holectypoida, and many of the Cassidulidae, the apical 
system consists of five minute radial plates, and one large, pentagonal, central 

