Prof. S. Loveii on the Stmcture of the Echinoidea. 377 



obstacle to cause compression or enlargement in width. On 

 the contrary, when a primary plate in the ambulacrum reaches 

 the margin of the corona, the suture which previously united 

 it with the next following plate separates, it becomes free, and 

 it moves out into the buccal membrane as if out of the mouth 

 of a river (fig. 2). At the same time a change takes place in 



Fio-. 2. 



Cidaris hjstrix, third ambulacrum : the ninth plate in b has scarcely 

 separated ; its fellow in a is already free and converted into a lamella. 



its form. The curved process on the inside of each plate, 

 which is more elevated the nearer we approach the peristome, 

 is absorbed when the plate separates, and disappears quickly. 

 The plate wears away at its lower margin, and its radiolar 

 tubercle diminishes ; but it increases greatly in breadth towards 

 the interradium, and also in depth, and thus becomes attenuated 

 into a lamella, or scale, which lies with its lower margin over 

 its predecessor. The pores thus come gradually to be situated 

 lower and lower, become drawn out transversely, and change 

 their position relative to the longitudinal axis. In the rows of 

 scales produced in this manner in the buccal membrane of the 

 Cidaridffi, the pair which most closely approach the mouth are 

 the oldest plates, the others, each in its order, have separated 

 themselves from the corona, and gradually increased the num- 

 ber of lamella; in each row. 



Ann. & Mag. N. Hist. Ser.4. Vol.x. 27 



