QnJg'rtonia, a new genus of Fossil Echimda. 161 



the various pieces are first united, and as the animal ad- 

 vances in life the consolidation goes on from below up- 

 wards, and at the same time the expansion of the shell takes 

 place by the deposit of new layers around the edges of the 

 plates. Small papulae also shoot out from the intervals 

 that formerly separated the different plates from each other, 

 thus obliterating in adult shells every vestige of distinct 

 plates. 



On the inner surface of the testa however, elevated ridges 

 still mark the former lines of separation, and on holding the 

 shell to a strong light the various pieces composing it become 

 apparent, owing to the transparency of the central parts 

 of the plates, while the margins are rendered thick and 

 opaque by the accumulation of testaceous matter thrown 

 out, and by which their union is ultimately effected. How 

 long the animal continues to live after it is completely 

 formed, we have not ascertained ; but from the great va- 

 riety in the size of the testa in such as we have met 

 with, we should think that, like fishes, they continue to 

 increase in bulk with their age. The largest Indian species 

 I have seen were nearly five inches in diameter, but I be- 

 lieve Echinus esculentus attains a still larger size on the 

 coasts of Europe. The large species which is very 

 common in collections from the Maldive islands, and 

 from the study of which much of my information is de- 

 rived, appears to live for a considerable time after the 

 different parts of the shell have been completely conso- 

 lidated, still progressing in bulk, but at a certain period, 

 which we should think corresponds with that at which 

 the augmentation of the animal becomes suspended, a new 

 set of actions then ensue, which are beautifully depicted 

 on the shell of the species in question. We learn from the 

 changes alluded to that these animals have an absorbent sys- 

 tem that performs a prominent part in their development and 



