276 ECHINODERMS OF THE BRITISH ISLES 



special outer layer, never with fine, anastomosing hairs. Second- 

 ary spines usually of the same shape as the primaries. Teeth with 

 or without keel. Mostly four sorts of pedicellariai present, 

 globiferous, tridentate, ophicephalous, and triphyllous. Spha^ridia 

 present. Spicules of tube-feet irregular or C-shaped. 



This great order, which comprises all the numerous recent 

 and fossil regular Echinoids, excepting the Cidarids, is divided 

 into three suborders, characterised mainly by the characters of 

 the dental apparatus and of the compound ambulacral plates. 

 Two of these suborders are represented in the British seas, and 

 the third no doubt will be found also, though not yet actually 

 known from there. 



Key to the suborders of the Diadematoidea. 



1. Teeth without a keel on the inside ; the epiphyses of the dental 



apparatus not joining above the teeth (Fig. 146, 2). In the 

 compound ambulacral plates the lowermost primary plate 

 but one is the largest (Fig. 155, 1). Tubercles perforate and 

 crenulate ....... I. Aidodonta 



Teeth with a keel on the inside. Tubercles not perforate . 2 



2. Epiphyses of the dental apparatus not joining above the teeth. 



In the compound ambulacral plates the lowermost primary 

 plate but one is the largest . . . .II. Stirodonta 



Epiphyses joining so as to form an arc above the teeth (Fig. 146, 1). 

 In the compound ambulacral plates the lowermost primary 

 plate is the largest (Fig. 155, 2) . . III. Camarodonta 



1 2 



Fig. 155. — Compound ambulacral plates. (From DanmarTc's Fauna.) 



1. The Diadematoid type, the middle or, in polyporous phites, tlie lowermost primary 

 plate but one being the largest. 2. The Echinoid type, the lowermost primary plate being 

 the largest. Diagrammatic. 



I. Suborder. Aulodonta Jackson. 



Test usually rather thin, sometimes flexible. Compound 

 ambulacral plates of the Diadematoid type, the middle or, in the 

 polyporous forms, the lowermost primary plate but one being 

 the largest. Primary spines without an outer layer, usually long 

 and fragile, hollow, mostly finely spiny, the spinules being more 

 or less distinctly arranged in spirals. Tubercles perforate and 

 mostly crenulate. Teeth concave, without a keel on the inside, 

 the epiphyses on the pyramids not joining above the teeth. 



