10 Trans. Acad. Sci. of St. Louis. 



Synopsis of the Orders, Families, Genera and Species 

 OF THE Palaeozoic Palaeechinoidea. 



The mistake of considering the variations in the different 

 parts of the corona of specific importance has led to the crea- 

 tion of many new species throughout the subkingdom of the 

 Radiates, which, in reality, are mere deviations of already ex- 

 isting species. It is a well established fact that no subking* 

 dom is so subject to variations as that of the Radiates. 

 Deviations from the pentamerous arrangement are the rule 

 rather than the exception. In the light of this knowledge 

 authors should be very chary about describing new species 

 with variable features as a basis. Any classification to be of 

 value must rest on features which are constant and typical. 

 Among the Palaeechinoidea the prevailing characters which 

 may be taken as a basis for classification are the following : — 



1. Number of columns in the ambulacra. 



2. Position and number of the ambulacral pores. 



3. Ornamentation of the plates. 



4. Imbrication of the plates. 



5. Apical system. 



6. General shape of the body. 



7. Geological position. 



Classificotion. 



Class ECHINOIDEA. 



Subclass Palaeechinoidea. 



Order A. CYSTOCIDAROIDA. 



Fam. I. Palaeodiscidae. 



Genus a. Palaeodiscus. 

 Fam. II. Echinocystidae. 



Genus a. Echinocystis, 



Order B. BOTHRIOCIDAROIDA. 



Fam. I. Bothriocidaridae. 



Genus a. Bothriocidaris» 



