CIDAEID^. 



left anterior and the odd interambulacral area. The actinal plates have 

 scarcely changed their size, showing that in Porocidaris Cobosi the resorption 

 of the actinal interambulacral plates is limited to that of the primordial, 

 of the first and part of the second plates. The ninth pairs of interambu- 

 lacral plates indicated in the preceding stage have become fully developed, 



Fig. 35. 



Porocidaris Cobosi. 



Fig. 36. 



as well as the tenth pairs. The eleventh pair of plates is indicated in the 

 odd posterior and the left anterior ambulacra. 



In the resorption of the actinal interambulacral plates, one of the plates 

 becomes triangular, Fig. 35, and disconnected from the adjoining ambula- 

 crum (PI. 11, figs. '2, 3), though when seen from the interior, Fig. 36, it 

 is rectangular (PI. 11, fig. //) and extends into the auricle, Fig. 37. 



With the resorption of the interambulacra and the flow of the ambulacra 

 towards the actinostome, some of the primary tubercles naturally disappear 

 and become resorbed as the ambulacral plate and its pore passes over to the 

 actinal system (PL 12, fig. .;), or the interambulacral plate becomes resorbed 



Fig. 37. Porocidaris Cobosi. 



and gradually cuts off a part of the primary tubercle (PI. 12, fig. 6'). In a 

 still later stage (PI. 12, fig. 7) the primary tubercles are moving towards 

 the actinal system. 



With the increasing age and size of the actinal system the number of 

 ambulacral plates to be liberated and flow over on the actinal system 

 gradually diminishes, as well as the addition of the irregular interam- 

 bulacral plates which do not reach the actinostome. 



