THE ECHINOIDEA 



309 



be a survival of the Orthopsidae. The apical plates form a single 

 circle, within which are five large anal plates around the anus (Fig. 

 XXII.}. 



FAMILY 2. DIADEMATIDAE. Diademina in which the ambulacral 

 plates at the ambitus are compound, and consist of three (Fig. XXIII.) 

 or more (Fig. XXIV.) primaries fused together with an occasional demi- 



Fio. XXIII. 



Compound ambula- 

 cral plates of 

 diadema hemisphaerica ; 

 tlii! simple diademoid 

 type of three primaries. 



Fio. XXIV. 



Compound ambulacral 

 plate of Pseudodiadema 

 hemisphaerica, containing a 

 demi-plate (<?). 



Fio. XXV. 



Compound ambulacral plates 

 of Placodiadema Michelini (after 

 Duncan) ; one formed of four and 

 the other of five components. 



plate (d). This family includes fourteen genera ranging from the Lias 

 to recent times, viz. Diadema, Gray ; Pseudodiadema, Desor (Figs. XXIII. , 

 XXIV.) ; Microdiadema, Cott. ; Diademopsis, Desor ; Hemipedina, "Wright ; 

 Echinodiadema, Cott. ; Pleurodiadema, Lor. ; Placodiadema, Dune. (Fig. 

 XXV.) ; Heterodiadema, Cott. ; Codiopsis, L. Ag. ; Magnosia, Mich. ; 

 Cottaldia, Des. ; Centrostephanus, Peters ; Helikodiadema, Gregory. 



FAMILY 3. DIPLOPODIIDAE. Diademina in which the ambulacral 



FIG. XXVI. 



Ambulacral plates of !>:/> 



/ra (after Duncan), showing the 

 biserial arrangement of the pore-pairs. 



Fio. XXVII. 



Compound ambulacral plates 

 of 1'iiiiin'. Sniithi; one plate 

 consists of three primaries, 

 and one of a central primary 

 ami two demi-plates (d) ; the 

 pore -pairs occur in oblique 

 series, \vith three pairs in each. 



plates are compound ; the pore-pairs are biserial either throughout the 

 area, or at least near the peristome (Fig. XXVI.). Genera Diplopodia, 

 M'Coy ; Phymechinus, Des. ; Asteropsis, Cott. ; Diplotagma, Schltiter ; 

 Plistophyma, Peron & Ganthier ; (?) Acanthechimis, Duncan & Sladen ; 

 M/i-ropyga, A. Ag. 



FAMILY 4. PEDINIDAE. Diademina in which the ambulacral plates 

 are compound and the pore-pairs are triserial (Fig. XXVII.). Genera 

 Pedina, Ag. ; Pseudopedina, Cott. ; Heterocidaris, Cott. ; Stomechinus, Des. : 



