Schuchert — Jackson on the Phytogeny of the Echini. 201 



" 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 have high hexagonal amhulacral 

 plates with the pores of the pore-pairs superposed. Each inter- 

 ambulacrum has a single plate ventrally, succeeded by two 

 plates in the next row. The peristome has a single row of pri- 

 mordial ambulacral plates which are like those of Bothrioci- 

 daris 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 young cidarids the genitals are large 

 and oculars small and exsert, unlike Bothriocidaris. . . . 

 The Cidaroida present distinctly a combination of Palaeozoic 

 with modern characters" (211-2). 



Order Cidaroida with 10 columns of simple ambulacral 

 plates and 10 of interambulacrals. Coronal plates rarely imbri- 

 cate. Represented in the Paleozoic by Miocidaris (1 species 

 in the Permian Zechstein) of Germany, and another in the 

 Millsap of Colorado. Order well represented from early Meso- 

 zoic time to Recent. Out of the Cidaroida was developed the 



Order Oentrechinoida, where the ambulacral plates are usu- 

 ally compounded of demi-plates. The stock arose in the Tri- 

 assic and continued to Recent. This order divides into 3 new 

 suborders : (1) Aulodonta (Triassic to Recent), with teeth of 

 the lantern grooved, and with epiphyses narrow and not meet- 

 ing in suture over the foramen magnum (Hemicidaridse, 

 Aspidodiadematidae, Centrechinidae, and Echinothuriidae) ; (2) 

 Stirodonta (Jurassic to Recent), with the teeth keeled and with 

 narrow epiphyses (Saleniidae, Phymosomatidae, Stomopneu- 

 stidae, and Arbaciidae) ; (3) Camarodonta (Cretaceous to 

 Recent) with keeled teeth and wide epiphyses meeting in 

 suture over the foramen magnum (Temnopleuridae, Echinidae, 

 Strongylocentrotidae, and Echinometridae). The last named 

 suborder is the most specialized of modern regular Echini. 



Order Exocycloida, or the irregular Echini, developed out 

 of the Stirodonta in the Jurassic and persists to Recent. Here 

 the periproct is always outside of the oculo-genital ring and 

 lies in interambulacrum 5. "Assuming a monophyletic origin 

 for the group, the three suborders present a striking series of 

 structural departures from the regular Echini from which they 

 doubtless originated. Considering the characters of the gi'oup 

 as a whole in brief, the compound ambulacral plates and peri- 

 stomal gills of the Holectypina and the auricles of that group 

 and the Clypeastrina, the existence of keeled teeth, where teeth 

 are known, and the presence of sphaeridia, are all characters 

 which unquestionably associate the Exocycloida with the Cen- 

 trechinoida and not with the Cidaroida, where these structures 



Am. Jour. Sci.— Fourth Series, Vol. XXXIV, No. 201.— September, 1912. 

 18 



