PAN AM 10 DEEP SEA ECHINI. 



This holds good mainly for older specimens. In younger stages the 

 outline of the test is modified by the projecting ocular plates, due to the 

 great development of the abactinal system in younger stages, Figs. 5, 6. 

 The ratio of increase of the ambulacral and interambulacral zones does not 

 vary greatly with age. 



In a young specimen of Porocidaris Milleri of 14 mm., Fig. 7, taking the 

 odd anterior ambulacrum for comparison, interradial plate 2 covers one 



ID 



14 mm. 



Fig. 6. Porocidaris Milleri. 



Figs. 7-10. Porocidaris Milleri. 



whole plate and two half plates, 3 covers two whole and two half plates, 

 4. tour whole and two halves, while 5 covers six and a half plates. We 

 find practically the same conditions in the odd ambulacra of specimens of 

 18 and 20 mm., Figs. 8, 0. Even when compared to a specimen of 28 mm., 

 Fig. 10, the number of ambulacral plates corresponding to the fifth inter- 

 ambulacral plate has only increased by one plate. 



