220 



PHYLUM ECHINODERMATA. 



always lies in the posterior interradius. But in other cases, in which 

 these marks are not so distinct or are not available, the orientation can 

 be determined, as well as confirmed in the preceding cases, by certain 



IV 



FIG. 163. SrissopsislyriferaForb. The plates of the shell spread out and viewed from the 

 oral surface (after Loven). I to V the radii numbered ; 1 to 5 the interradii numbered. 

 In the interradii 1, 4, 5, the three plates next the peristome are numbered to show corre- 

 sponding plates (note asymmetry in 1 and 4), a and b mark the rows in each radius 

 and interradius. In interradius 5, 1 marks the plastron and is placed on the labrum, 2 

 (st) the sternum, and 3 (ep) the episternum. The apical system is shown in 2, the inter- 

 radius of the madreporite. an anus. 



peculiarities of a very remarkable character in the peristomial ambulacral 

 plates. These peculiarities were discovered by Loven, and the statement 

 of them constitutes Loven's law. In all Echinoids, except some Pour- 

 talesiidae, the ambulacral plates which border the peristome of the bivium 



