Schuchert — Jackson on the Phylogeny of the Echini. 259 



five ambulacral areas and from one to fourteen columns of 

 plates in each interambulacral area. New coronal plates are 

 formed at the ventral border of the five ocular plates, ambu- 

 lacral pores pass through ambulacral plates, rarely (clypeas- 

 troids) in part between plates. The peristome in all but 

 the Exocycloida bears from one to many rows of ambu- 

 lacral plates, with or without non-ambulacral plates. There 

 are five oculars (apparently in part or wholly wanting 

 in some of the Pourtalesiidae), and five genitals or fewer, 

 the whole being fused into a mass in certain types of 

 Exocycloida. The genitals typically have each one or more 

 pores as exits of the five interradially situated reproductive 

 glands. In addition, typically, madreporic pores exist in gen- 

 ital 2, but are not recognizable in most Palaeozoic forms. The 

 periproct is more or less plated, situated within the oculo- 

 genital ring, or in irregular types outside of that area ; the 

 anus is in the periproct. The masticatory lantern is composed 

 of forty pieces (or clypeastroids thirty pieces) ; it is wanting in 

 adult spatangoids. Respiratory organs consist of Stewart's 

 organs, peristomal or ambulacral gills. Locomotion is effected 

 by ambulacral feet or by spines, or both" (200). 



A key to the classification of the Echinoidea is given on 

 pages 201 to 208. Other keys to the species of Paleozoic 

 Echini are given under the systematic descriptions. 



Ancestors of Echini. — The author states that Echini "make 

 no close approach to other classes of the Echinodermata. . . . 

 What the ancestor of the Echini as a class was is unknown, 

 but it might fairly be sought amongst the Cystoidea" (200). 



Basis of classification. — All Echini recent and fossil are 

 classified by Jackson on the basis of "the structure of the adult 

 and the development of the same. ... no single character has 

 been followed." The characters taken into consideration are : 

 the ambulacrum, interambulacrum, coronal imbrication, basi- 

 coronal plates, ventral resorption of corona, ocular and genital 

 plates, periproct, peristome, Aristotle's lantern, perignathic 

 girdle, spines and tubercles, gills and sphaeridia. " The relative 

 value of these parts naturally differs in different groups of the 

 Echini " (199). 



The protechimcs. — " 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 structure with that seen in the very young 

 of all geologically later Echini known and the youthful charac- 

 ters retained at the ventral border in the adults of many types " 

 (208). 



" Each interambulacrum of Bothriocidaris consists of a single 

 column of plates, which is represented by a single plate at the 

 ventral border of the interambulacra in the young of all other 



