DERMATODIADEMA HORRIDUM. 



67 



proportion than in older specimens (PL 27, figs. 5, 6). The plates immedi- 

 ately surrounding the anus are placed on edge and send out long calcareous 

 threads (PI 27, fig. 1). 



The buccal plates of the actinal system occupy a proportionally greater 

 space of the actinal system in young specimens than in older ones. In a 

 specimen of 7.3 mm. in diameter with six and seven interambulacral plates, 

 Fig. 101, there is only a narrow belt between the distal edge of the buccal 

 and the coronal plates (PI. 25, fig. 1) filled with small irregularly shaped 

 calcareous plates. In somewhat older specimens 10 and 10.5 mm. in 



Figs. 102-106. D. hoekidum 



diameter the belt of small plates had increased in width (PI. 25, figs. ^, 3j .^), 

 and still more in larger specimens (Pis. 25, fig. 5 ; 26, fig. 1). The ambu- 

 lacral pores also assume with increasing size a more central position in the 

 buccal plate. 



There seems to be a regular sequence in the order of appearance 

 of the miliaries in the angles of the interambulacral plates, as is shown 

 in the accompanying figures of the fifth interambulacral plate from the 

 actinal system in specimens varying in size from 7.3 mm. to 20 mm. Figs. 

 102-106. 



