12 



PANAMIC DEEP SEA ECHINI. 



In a specimen of 30 mm., Fig. 29, the position and proportions of the 

 actinal interanibulacral plates is about as in the previous stage. The 

 seventh pairs of plates are fully developed, and the eighth pairs are much 

 more advanced than in the preceding stage. 



As Gon'wcidark canalkulcUa and Dorocidaris papiila/a have been shown to 

 have interambulacral primordial plates in very young stages, we may safely 



18 mm. 



Fig. 32. PoRocinARis Cobosi. 



.assume that they have already been resorbed in a young specimeu of 

 P. Cohosi of 8 mm,, Fig. 30, and that parts of the first and second inter- 

 radial plates have likewise disappeared (Pis. 10, figs. ^, ,T; 12, figs, i, 2). 

 Comparing this stage, having fifteen pairs of ambulacral plates, one of the 

 sixth pair of interambulacral plates in each area, and two series of buccal 

 j)lates with a somewhat older one. Fig. 31. of 10 mm. (PI. 12, fig. lO), 

 having three series of buccal plates, we can trace the extent of the resorp- 

 tion which lias taken place. In the younger specimen, beginning with 



