260 Schuchert — Tackson on the Phylogeny of the Echini. 



Echini. . . . there is ample proof that the interambulacrum 

 begins with a single plate, as shown by Lov^n (1874), and 

 Mortensen (1903). . . . This structure with less evidence I cor- 

 related ( L896) as a stage in development with the single column 

 of plates in Bothriocidaris, Darning it the protechinus stage. 

 AsPalaeozoic types with many columns of interambulacral plates 

 begin at the ventral border, the young, with a single plate rep- 

 rescntin<«; a single column, and later add their several columns 

 during development, it seems that Bothriocidaris throws great 

 light on the numerous columns there existent " (210). 



Order Bothriocidaroida. — Of Echini the oldest and most 

 primitive order is the Bothrioeidaroida, found in the Middle 

 Ordovician of Esthonia. The only genus, Bothriocidaris, has 

 3 very small species, with 10 columns of hexagonal ambulacral 

 plates, each with a pair of centrally placed podial openings, and 

 but 5 columns of interambulacrals which may have small spines. 

 Plates not imbricate. Periproet within the oculo-genital ring, 

 which consists of 5 very large oculars and 5 very small gen- 

 itals. Jaws present. It is out of this stock, the protechinus 

 stage, that all regular Echini have evoluted as follows : 



Later Echini. — " The feature of Palaeozoic Echini is that 

 they have more than two columns of plates in each interambu- 

 lacral area. This is true of all known forms excepting Bothri- 

 ocidaris and Miocidaris as far as the latter occurs in the upper 

 Palaeozoic. Gregory (1897), Sollas (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 types all have two columns of inter- 

 ambulacral 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 iind 

 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 except- 

 ing as seen in senescence (Perischocidaris), and in the little 

 known Tetracidaris of the Cretaceous. I, therefore, consider 

 the Echini usually classed as the Euechinoida, with a geolog- 

 ical range from the Lower Carboniferous to Recent inclusive, 

 and comprising the orders Cidaroida, Centrechinoida, and Exo- 

 cycloida as next related to Bothriocidaris. This view is based 

 on structure and development. I am well aware of the inter- 

 vening [greatj 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. . . . 



