176 SYNOPTIC COLLECTION. 



The ambulacra arise from four plates (PI. 321, fig. 2, a, 

 b, a', b' ; fig. 3, ambulacrum enlarged, a, b, a 1 , &'). This 

 would indicate that in the development of Melonites the 

 adult condition of Bothriocidaris, in which the ambulacra 

 consist of two rows of plates, has been skipped by the law 

 of acceleration in development, and that the four-plate 

 st.ige is homologous with the adult of Oligoporus, as 

 pointed out by Jackson, or it may be, as suggested by this 

 investigator, that the ambulacrum of Melonites starts with 

 two plates which might be seen in the young could such 

 specimens be obtained. If this is the case these plates 

 have been resorbed during the growth of the animal. 



New rows of plates are added between those already 

 formed (PI. 321, fig. 3, c, d> and e,f), so that each ambu- 

 lacrum becomes more complex than any so far described. 

 A cross section of an ambulacrum (fig. 4, magnified 2 

 diameters) shows the relative thickness of the four ambu- 

 lacral plates, and also proves the fact that the holes pass 

 diagonally and not straight through the shell. The dotted 

 portions of the pores are reconstructions, these parts not 

 being clearly shown in the section. 



The interambulacrum (No. 320; also PI. 321, figs. 2, 

 5) apparently arises from two plates as seen in the speci- 

 mens (No. 320, the dotted lines beginning in two plates; 

 also seen in PI. 321, figs. 2, 5). According to Jackson it 

 is most probable that this area originates in one plate, 

 which later was resorbed. With the growth of the animal 

 the interambulacrum becomes complicated by the addi- 

 tion of a number of rows (fig. 5). This diagram repre- 

 sents the ideal arrangement of plates in one interambula- 

 crum as determined by prolonged and critical observation 

 of a large number of specimens. The theoretical plate /' 

 is included in the figure to indicate all the possible plates 

 the interambulacrum had at any period of growth. This 

 plate is resorbed in the adult, as already stated. Eight 

 rows are found most commonly. As these rows approach 

 the anal area the mechanical necessity of the case com- 



