CHAPTER XV. 



THE ECHINOIDEA. 1 



CLASS III. ECHINOIDEA. 



SUB-CLASS 1. REGULARIA ENDOBRANCHIATA. 

 Order 1. Bothriocidaroida. 

 ,, 2. Cystocidaroida. 

 ,, 3. Cidaroida. 

 4. Melonitoida. 

 5. Plesiocidaroida. 



SUB-CLASS 2. REGULARIA ECTOBRANCHIATA. 

 Order 1. Diademoida. 

 Sub-Order 1. Calycina. 

 2. Arbacina. 

 3. Diademina. 

 4. Echinina. 



SUB-CLASS 3. IRREGULARIA. 



Order 1. Gnathostomata. 

 , Sub-Order 1. Holectypina. 

 2. Clypeastrina. 

 Order 2. Atelostomata. 

 Sub-Order 1. Asternata. 

 ,, 2. Sternata. 



KNOWLEDGE of the Echinoidea or Sea Urchins is more complete 

 than that of any other of the classes of Echinoderma. The 

 class is widely distributed at the present day, members of it 

 living in seas of all parts of the world and at all depths. It is of 

 great antiquity, and fossil Echinoids occur abundantly in rocks of 

 all periods since the Devonian, while a few are known from that 

 system and from the Silurian. Moreover, as the classification of 

 the Echinoids depends either upon the skeleton, or upon structures, 

 the characters of which can be inferred from those of the skeleton, 

 the systematic position of a fossil can generally be determined with 

 as much accuracy as that of a recent specimen. 



1 By J. W. Gregory, D.Sc. MS. closed at end of 1896. 



