THE ECHINODERMATA 271 



tion named). This has been made the basis of a 

 classification into " Palechinoidea " and " Euechinoidea," 

 but as a morphological classification this leaves Tetvaci- 

 daris in an anomalous position; as a genetic classification 

 it is unsatisfactory, as it unites a number of divergent 

 Palaeozoic stocks, of which one was apparently more 

 closely related to Mesozoic regular echinoids than the 

 others. The following classification is based on that of 

 Martin Duncan, with modifications taken from the 

 classifications of Professor J. W. Gregory and Dr. R. T. 

 Jackson, neither of which is fully adopted. 



CLASS: ECHINOIDEA. 

 SUB-CLASS : Regularia (Endocyclica). 



Order i. BOTHRIOCIDAROIDA. With a single column 

 of interamb plates. Only genus Botknocidaris (Fig. 78, a), 

 Ordovician of Baltic Provinces, the only known Ordo- 

 vician echinoid. 



2. ECHINOCYSTOIDA. Two aberrant forms, morpho- 

 logically exocyclic, though not related to the Irregulares. 

 Echinocystis and Palaodiscus, Silurian of Shropshire. 



3. MELONECHINOIDA. A series in which both amb 

 and interamb columns tend to increase in numbers ; no 

 large tubercles, only granules with small radioles. 

 Silurian to Permian. Palaechinus (Sil.-Carb., Fig. 78, b), 

 Melonechinus (Carb., Fig. 78, d). 



4. CIDAROIDA. A series in which the ambs have never 

 more than two columns, but in Palaeozoic genera and 

 in the Barremian (Tetracidaris) the interambs have more 

 than two. Even where there are only two interamb 

 columns the interambs are many times wider than the 

 ambs. Large tubercles and radioles on the interambs 



