PLATE ARRANGEMENT IN OLIGOPORUS COREYI. 



191 



Melonites multijwrus 

 at ambitus. 



Melonites multq^orus at ven- 

 tral border. 



Oligoporus danx at ambi- 

 tus. 



genera, for in the interambulacrum a corresponding progressive develop- 

 ment occurs as well (see classification in table facing page 242). 



The interambulacrum of Oligoporus coreyi (plate 6, figure 25) terminates 

 ventrally in two 



plates, 1 and 2, ^ o!. h 



as shown in 

 both areas of 

 the figure, just 

 as previously 

 seen in Melonites mul- 

 tipor as. As this speci- 

 men is viewed from 

 the inside, the left ad- 

 ambulacral column 

 appears on the right 

 side of the figure, et- 

 cetera, and the rela- 

 tive position of the 

 areas appears for the 

 ■same reason in inverse 

 order to that shown in Me- 

 lonites multiporus (plate 2, 

 figure 2), which is viewed 

 from the outer or distal side 

 of the corona. This fact 

 must be borne in mind in 

 considering the description 

 of the figure, as otherwise 

 features which are really 

 perfectly normal, accord- 

 ing to the law of growth, 

 would appear abnormal or 

 reversed. 



Plates numbers 1 and 2 

 at the ventral border form 

 the basis of the two adam- 

 bulacral columns of pentag- 

 onal plates, as in Melonites. 

 Plate 1 of the right-hand 

 area is mutilated on its ventral aspect, so that it presents a jagged out- 

 line ; but plate 2 is entire on its ventral aspect. This plate has an 

 inclined face leading upward from the ventral to its median portion, 



Oligoporus coreyi at ventral bor- 

 der. 



Palseechinus gigas at ambitus. 



Rhoechinus elegans at ambitus. 



Bothriocidaris pahleni at ambitus. 



Figure \.— Relations of the ambulacral Plates. 

 The diagram illustrates the relations of the ambulacral 

 plates at the ambitus or the ventral border and ambitus 

 of the several included genera. Compare with figures in 

 plates and figures 3 and 4, page 234, see also page 142. 



