Introductio7i to Animal 2forphology. 147 



no circum-oral gills, and the spines are large and clavate ; 

 ■anal plates ten. 2. Salenidae — extinct, (except S. rari- 

 spina), Jurassic and cretaceous forms with eleven anal plates. 

 3. Diadematidas — perforated, crenulated tubercles ; spines 

 usually hollow ; the ambulacral area with perforate tubercles ; 

 peristome with notches. 4. Arbaciidae — tubercles smooth, 

 imperforate ; no sutural pores ; periproct closed by four plates. 

 5. Echinidae — spine-warts and sutural pores as last ; periproct 

 of many plates ; body circular or pentagonal ; pores in three 

 or four pairs on each plate; including Temnopleurinse, with oral 

 notches shallow, and no ocular plate reaching the periproct ; 

 and Triplechinids, with two ocular plates reaching the peri- 

 proct, and the oral notches deeper than broad. 6. Echino- 

 metridae — pores in five or six pairs ; body elliptical ; including 

 Heterocentrotus, with club-like spines ; Colobocentrotus, with 

 the dorsum of the test covered with pavement-like spines, &c. 

 Sub-order 2. Exocyclica — the anus not in the centre of 

 the apical apparatus: there are five families: — i. Galeri- 

 tidse — genital pores four ; ambulacra ending in an apical 

 point; including the toothless Echinoneus. 2. Dysasteridae — 

 Mesozoic, Avith the ambulacra converging to two apical 

 points ; three anteriorly forming a trivium ; with three 

 ocular and four genital plates, two posteriorly (bivium), with 

 two ocular and no genital plate. 3. Clypeastridae — ambulacra 

 petaloid, often not distinct towards the margin ; mouth 

 central, with teeth ; anus inferior ; spine-warts notched and 

 perforate ; shell wall with duplicatures, pillars, and chambers ; 

 madreporiform plate central ; the shell may be inflated 

 and the jaws moveable on the auricles (Clypeastrinas), or 

 fastened thereto, and the shell flat (Laganinae) ; the shell 

 may be lobed, or pierced by large holes, and the am- 

 bulacral rows branched (Melittinae). 4. Cassidulinidas — 

 mostly cretaceous and eocene ; toothless, with petaloid 

 ambulacra, imperforate and entire spine-warts ; genital pores 

 five; anus posterior or inferior. 5. Spatangidae — heart- 

 shaped, toothless, with an anterior reniform mouth, and inter- 

 rupted ambulacra ; bivium embracing a sternal field ; semitae 

 may be entopetalar and sub-anal, not peripetalous (Echino- 

 cardium, Lov^nia), or with a peripetalous area (Breynia) ; 



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