300 THE ECHINOIDEA 



SOB-CLASS 1. REQULARIA ENDOBRANCHIATA. 



Mouth and anus at opposite poles. Anus surrounded by the apical 

 system of plates, when they are present. No external gills. 



ORDER 1. Bothriocidaroida, Schmidt. 



Loven has shown that in many young Echinoids the interambulacrum 

 begins and ends with a single plate. In the oldest known Echinoids the 

 whole interambulacrum consists of a single vertical series. The Echinoids 

 in question are two species from the Ordovician rocks of Esthonia. They 

 belong to the genus Bothriocidaris, Schmidt. 



Bothriocidaris has a small test, on the top of which is an apical 

 system (Fig. XIV. 2), composed of a ring of five large ocular plates, in 

 the angles between which are five small imperforate genital plates. 

 Each ocular plate has two pores. The anus is in the centre of the 

 apical system, and the periproct is covered by six or eight anal plates. 

 The test is mainly formed by the ambulacra. Each ambulacrum consists 



. Bothriocidaris Pahleni, Schmidt ; Ordovician, Russia. 1, from the side ; 2, apical system ; 

 3, peristoiniiil plates. The interambulacral plates are shaded. 



of two vertical series of hexagonal plates, each perforated by one or two 

 pore-pairs. The interambulacra are narrower than the ambulacra, and 

 consist of a single series of plates which do not extend to the peristome, 

 from which they are cut off by the expansion of the peristomial ambulacral 

 plates (Fig. XIV. 3). The most recent description of the genus is by 

 Jaekel (42). 



ORDER 2. Cystocidaroida, Zittel. 



Echinoidea Regularia Endobranchiata with test ovoid, flexible. No 

 apical system of plates. Madreporite and anus (when present) open in- 

 dependently in the posterior interambulacrum. Mouth central. Ambu- 

 lacra of low, closely packed plates. Interambulacra broad, of numerous, 

 thin, and irregularly arranged plates, bearing short spines. A masticatory 

 apparatus present. 



FAMILY 1. PALAEODISCIDAE. Cystocidaroida with depressed, discoid 

 body. The ambulacral plates are biserial, crowded, and narrow ; on the 

 oral surface they are not perforated by pores, but the podia pass out 



