238 PHYLUM ECHINODERMATA. 



The classification of the Echinoidea is at present under revision 

 at the hands of Mortensen and others (op. cit.). Pending the 

 completion of this work, we have after some hesitation decided 

 to adopt, in its main features, the classification propounded by 

 Duncan (op. cit.) in 1891. It has two considerable advantages : 

 it holds the field, having been adopted by Zittel, Delage and 

 Lang in their valuable textbooks ; and it is extremely simple, 

 introducing the smallest possible number of new terms. 



The system is as follows : 



Order 1. PALAEECHINOLDEA. 



2. EUECHINOIDEA. 



Sub-order 1. Cidaroida. 



2. Diadematoida. 



Section 1. Streptosomata. 



,, 2. Stereosomata. 



Sub-order 3. Holectypoida. 



4. Clypeastroida. 



5. Spatangoida. 



Section 1. Asternata. 



2. Sternata. 



Order 1. PALAEECHINOIDEA.* 



With only one or with more than two vertical rows of plates in 

 each of the five interradii, and with two or many vertical rows of 

 simple or compound plates in each of the five ambulacra. 



This sub-class comprises exclusively extinct and for the most 

 part palaeozoic "t forms. To the characters mentioned in the 

 definition the following may be added. The peris tome is in the 

 middle of the oral surface. Jaws are present. The plates may 

 or may not overlap. The anal area is either within the apical 

 system, or outside it in the posterior interambulacrum. Echino- 

 cystites alone is known to be exocyclic. The sub-class dates from 

 the Upper Cambrian (Bothriocidaris). 



The most important genera are as follows : 



Bothriocidaris Eichw., interambulacral plates in one row, Upper Cam- 

 brian. Echinocystites W. Thorns., exocyclic, the anus and madreporite 



* K. A. Zittel, Handbuch der Palaeontologie, Leipzig, 1880 ; also A. 

 Agassiz, op. cit., P. Martin Duncan, op. cit. 



f Tiarechinus is from the Trias, and Tetracidaris, which is here placed 

 "with the Euechinoidea among the Cidaridae, but is by some regarded as 

 belonging to the Palaeechinoidea is from the Cretaceous. 





