SYSTEMATIC CLASSIFICATION OF ECHINI. ■ 211 



7-9) all of the genitals separate the oculars and meet the interambulacra; interambulacral 

 plates bear spines. These several species occur in the Ordovician of Russia (p. 238). 



The feature of Palaeozoic Echini is that they have more than two columns of plates in each 

 interambulacral area. This is true of all known forms excepting Bothriocidaris and Miocidaris 

 as far as the latter occurs in the upper Palaeozoic. Gregory (1897), SoUas (1899), and others 

 have assumed that the most primitive form of Echini had many columns of interambulacral 

 plates in an area, and several authors have considered Palaeodiscus as the most primitive 

 known type. On this basis evolution would entail a loss of such parts, as our modern tj^pes 

 all have two columns of interambulacral plates in an area. The evidence of development and 

 adult structure is opposed to this view. At the ventral border of the young of all known modern 

 types, and at the ventral border of the adult where not removed by resorption, we find a single 

 primordial plate in each interambulacral area succeeded in the second row by two plates. There 

 is no evidence in development of a larger number of columns dropping out to two in any known 

 living form, or indeed, in any fossil form excepting as seen in senescence (Perischocidaris, 

 Plate 65, figs. 1, 2; Plate 67, figs. 2, 3), and in the little known Tetracidaris of the Cretaceous. 

 I, therefore, consider the Echini usually classed as the Euechinoida, with a geological range 

 from the Lower Carboniferous to Recent inclusive, and comprising the orders Cidaroida, Cen- 

 trechinoida, and Exocycloida as next related to Bothriocidaris. This view is based on structure 

 and development. I am well aware of the intervening geological gap, but can only appeal to 

 the rarity of all forms in the Silurian and Devonian to account for the absence of intermediate 

 types. Wliile the Palaeozoic orders Echinocystoida and Perischoechinoida are not considered 

 as in the direct line of ancestry of modern forms, they yet show many primitive characters, 

 especially as regards the peristome and lantern, which can well be compared with the condition 

 in the young of later types. 



The order Cidaroida is placed as directly derived from the Bothriocidaroida without 

 known intermediate forms. The Cidaridae as regards the structure of the young and adult 

 are the least removed from Bothriocidaris of any known echinoid, living or fossil. The young 

 (Plate 2, figs. 1-3, 6) have high hexagonal ambulacral plates with the pores of the pore-pairs 

 superposed. Each interambulacrum has a single plate ventrally, succeeded by two plates in 

 the next row. The peristome has a single row of primordial ambulacral plates which are like 

 those of Bothriocidaris excepting that in that type there are two peristomal rows. The base 

 of the corona has not yet been resorbed, exactly like adult Bothriocidaris. In j'oung cidarids 

 the genitals are large and oculars small and exsert, unlike Bothriocidaris. The young lantern 

 is inclined (Plate 2, fig. 17), with deep foramen magnum, long interpyramidal muscles (Loven, 

 1892) as in the Perischoechinoida, and the lantern muscles are attached directly to the basi- 

 coronal interambulacral plates without apophyses (Loven, 1892), as is the apparent condition 

 in the Perischoechinoida (text-fig. 221, p. 193). 



