SYSTEMATIC CLASSIFICATION OK K< HIM 201 



In Palaeozoic types the given characters, especially of the peri-tome, may be known in only 

 one or a few cases, and further study may require modification in detail-. A 



table is given (p. 209) that presents in a tree form the relation- :i> iindcrntood, ami following 

 this, as briefly as possible, a consideration of each of the several division.-, the full. 

 being given of the Palaeozoic forms. Post-Palaeozoic genera of the regular Echini, and tin- 

 families of the Exocycloida are not included. 



Key to the Classification of the Echinoidea. 



I. Two columns of high hexagonal plates in each ambulacra! urea, and one column of plate* in r li int.-r- 

 ambulacral area .................... Onlcr HOTUUKX IDAUOIO S 



With characters of the order ................ Family H.HIIKI.M IIMHIIMI 



Ambulacra! pores superposed in the center of plates. I'lates not imliricate. Primordial ambulacra! 

 plates around mouth in the peristome. Primordial intcramluilacral plates in basicoronal row. 

 Base of corona not resorbed. Oculars very large, genitals very small. Pcriproct with many 

 small plates. Peristome with two rows of ainhulacral plate^ only. Jaws. 



Bathriociitarit (three special). 



II. Two or more columns of plates in each ainhulacral area, and two <>r more columns of platen in cmch inicr- 

 ambulacral area. 



A. Two columns of simple or compound plates in each ainhulacral ami. and two colmiui-> of platnt in 

 each interamhulacral area (for A A, see p. J05). 



B. Periproct within oculo-genital ring (for BB, sec p. 204). 



C. Ainhulacral plates low, simple; two (in one genus, Tetracidark partly four) columns of 

 plates in each interamhulacral area. Coronal plates rarely imbricate ( Miocidaris'. Prim- 

 ordial ambulacral plates around the mouth in the peristome. Primordial interambulacral 

 plates resorbed. Base of corona resorbed. Oculars all exsert, or becoming insert in the 

 sequence V, I, IV, II, III, or V, I, IV 7 , III, II. Periproct covered with many thick plates. 

 Peristome with many rows of ambulacral and interradial non-ambulacral platen, or rarely 

 ambulacral plates only. I-antern erect, teeth gnxned, foramen magnum very shallow; 

 epiphyses narrow. No pits in the top of pyramids. Pyramids with ridges on lateral wiop. 

 Perignathie girdle consisting of apopli\scs only. Stewart's organs pn-*-nt. Primary spinet 

 with a cortical layer. Primary tubercles perforate. Sphaeridia absent. 



Order < IDAKOIhA 



With characters of the order. (Onl.\ one assured genus of this family <vurs in the Palaeo- 

 zoic, therefore other genera are omitted.) . . Family CIDAI 



Interambulacral plates beveling over the ambulacral on the adraclial sin 

 species only of this.order and genus are definitely known in the Palaro? 



Miiicidarii. 



CC. Ambulacral plates compound, rarely simple; two columns of plates in each intrramhulacral 

 area. Coronal plates not imbricate or imbricate. Primordial ambulacral plates around the 



