208 ROBERT TRACY JACKSON ON ECHINI. 



Six columns of plates in each ambulacral area. Five columns of plates in each 

 interambulacral area. Plates apparently iniliricating moderately. Eccen- 

 tric primary tubercles on certain adradial plates, with secondary tubercles 

 on the same and alone on other interambulacral plates. 



Perischocidaris (one species). 



Four columns of plates in each ambulacral area. Eleven to thirteen columns 

 of plates in each interambulacral area. Plates strongly imbricating. Small 

 primary with secondary spines and tubercles on interambulacral plates. 



Proterocidaris (one species). 



Eight to si.xteen columns of plates in each ambulacral area. Three to se\'en 

 columns of plates in each interambulacral area. Plates strongly imbricat- 

 ing; plates all of uniform size. Secondary spines and tubercles only. Test 

 elliptical, obovate, or spherical Lepidcsthes (ten species). 



Four to si.x columns of plates in each ambulacral area. Five to six columns of 

 plates in each interambulacral area. Plates strongly imbricating. .Xmbu- 

 lacral plates large ventrally, small dorsally; interambulacral plates dorsally 

 very large in adambulacral columns, smaller within. Eccentric primary 

 spines and tubercles with secondaries on dorsal adambulacral plates, and 

 secondaries onl^' on interambulacral plates of dorsal median columns. 



Pholidoeidaris (four species). 



Twenty columns of plates in each ambulacral area. Three columns of plates 

 in each interambulacral area. Plates of uniform size, imbricating strongly. 

 Small central primary spines and tubercles with secondary spines and 

 tubercles on ambulacral and interambulacral plates. Teeth distally serrate, 

 a unique character Meekechinus (one species). 



Having considered the systematic relations of the Echini in the form of a key and in a 

 genealogical table I will give briefly the reasons for the several steps which will be more com- 

 pletely gathered from the morphological studies, and from the systematic description of Palaeo- 

 zoic Echini. The geological distribution of the several groups of Echini as given in the table 

 (p. 209) is taken from the systematic part of this memoir excepting the groups of the Centre- 

 chinoida and Exocycloida which are taken from Duncan (1889a). 



The most primitive type of Echini, I believe emphatically, is Bothriocidaris. This view 

 is based on the simplicity of its structure, and especially on the close comparison of this struc- 

 ture with that seen in the very young of all geologically later Echini known and the youthful 

 characters retained at the ventral border in the adults of many types. In Bothriocidaris 



' I differ from Duncan in regard to the Hemicidaridae which I ilalf from the Trias whereas Duncan (lSS9a, p. 50) says 

 that Hemicidaris occurs in the Zechstein. Dr. Bather (lOOQa, p. 250) expresses liimself very positively that none of tlic 

 Ectobranchiata appear before the Trias and I prefer to follow this most accurate authority. I differ also in extendinfi; 

 the Holectypina only to the Eocene, as the Recent Pygastrides (Duncan, 1889a, p. 138) is very doubtful. 



