40 



W. KEEPING ON THE PALEOZOIC ECHINI. 



thuridae, that "they differ to the widest extent compatible with 

 belonging to the same suborder," might, it seems to me, apply with 

 greater force to the Perischoechinidse. Snch was Romer's opinion, 

 who ranked them as a separate suborder. 



I append the following classification of the Echinoidea. 



Order Echinoidea*. 



2 rows of plates in each area. 

 Suborder Eciiinida. 



More than two rows 



of plates in the 



interambulacral areas. 



Suborder Perischo- 



echinida (McCoy). 



Plates not 



imbricating; 



test rigid. 



Section Stereo- 



dermata t. 



Vent sur- 

 rounded by 

 the apical 



Subsection 



Endocyclica 



(Wright). 



Plates imbrica- 

 ting; test flexible. 

 Section Echino- 



tkuridcB 

 (W. Thomson). 



Tent not 



surrounded by 



the apical 



plates. 



Subsection 



Exocyclica 



(Wright). 



Plates not 



imbricating. 



Section 



Tessellata. 



Plates 

 imbricating. 



Section 

 Lepidermafa. 



Appendix. 



Since writing the above I have had an opportunity of examining 

 cursorily some specimens in the Jermyn-Street Museum. Amongst 

 them is a giant Echinus, from the Carboniferous Limestone of Derby- 

 shire, crushed and much disarranged. It covers an area of about 1\ 

 by 6| inches. Traces of the five ambulacra are seen converging to 

 what is, I believe, the mouth, of which aperture no trace is now left. 



The interambulacral plates are extremely numerous, mostly hexa- 

 gonal, but some (the marginal rows) pentagonal ; the plates have 

 suffered much from weathering, so that only a few of them show 

 that they were uniformly covered with small imperforate granules 

 surrounded by a faint ring. There must have been from 10 to 15 

 ranges of these plates in each interambulacral area. 



The ambulacral areas are very broad, and are composed of nume- 

 rous very irregular plates, each perforated by a pair of pores. There 

 are at least five rows of these plates ; and certain appearances suggest 

 that there may have been as many as 8 or 10 ranges, as in the 



* For definition see Huxley ' Introduction to Classification of Animals/ 

 p. 128. 



t arepeos, stiff, fixed. 



