214 



ECHINODERMATA EOHINOZOA 



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, D). It is evident from the inspection of any Sea- 



X 



III III 



FIG. 344. 

 Apical systems of A, Palceechinus. C, Salenia. D, Pdtastes, enlarged. The lAnib 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 Exocydica the basal plates, may be in contact at their sides, forming 

 a compact si/stem (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. 3 4 5, C). 

 When the two pos- 

 terior ambulacra 

 (bivium) do not 

 terminate at the 

 summit in line 

 with the other 

 three (trivium),&ud 

 are surmounted by 

 radials placed far 

 posteriorly, the 

 system is said to 

 be disjunct or dis- 

 connected. The pos- 

 terior radials are 



d^r1S;::"^;';: m If ;^S"? i " 4S?^ B ' Holectypus ' c ' Hyl - then separated 



from the postero- 



lateral basals by a number of interambulacral plates intercalated along the 

 dorsum (Fig. 345, A). 



In the Clypeastridae, Holectypoida, and many of the Cassidulidae, the apical 

 system consists of five minute radial plates, and one large, pentagonal, central 



FIG. 345. 



