OOLITIC ECHINODERMATA. 



399 



to the mouth, and surrounded by the five perforated genital and the five ocular 

 plates. Mouth always armed with five powerful calcareous jaws, formed 

 of many elements disposed in a vertical direction. 



Echinoidea, exocyclica. 



B. Test sometimes circular and hemispherical, oftener oblong, pentagonal, 

 depressed, clypeiform or discoidal ; mouth central or excentral ; vent ex- 

 ternal to the circle of genital and ocular plates, never opposite the mouth, but 

 situated in diflferent positions in relation to that opening : four of the genital 

 plates are generally perforated. The mouth is sometimes armed with jaws, 

 but is oftener edentulous. The jaws are disposed in a more or less horizontal 

 direction. 



The structure of the ambulacral areas and poriferous zones, the form, 

 number, and arrangement of the tubercles and their spines, the presence or 

 absence of fascioles or semitae, the size and form of the elements of the apical 

 disc, and the position of the anus, afford collectively good characters for de- 

 fining the genera. 



The minute details in the structure of the plates ; the size, form, and 

 number of the tubercles on each ; the form and arrangement of the pores in 

 the zones ; their proximity or remoteness from each other ; the general out- 

 line of the body, which has only certain limits of variation ; the character of 

 the sculpture on the plates ; the form of the areolas ; the greater or less pro- 

 minence of the base ; the size of the tubercle ; the presence or absence, the 

 size and arrangement of the granules forming the areolar circle ; the com- 

 pleteness or incompleteness of the same ; the width of the miliary zone, the 

 number and size of the rows of granules composing it ; tfce length of the spines; 

 the form of their stems ; the character of the sculpture on them ; the size of 

 the head, and the prominence and milling of the ring, — are all details of struc- 

 ture which individually and collectively afford good specific characters, as 

 they are persistent details which are more or less developed on every consider- 

 able fragment of the test and spines of the Echinoidea. 



Taking these characters for our guidance,! have grouped the genera, already 

 so numerous by the discovery of extinct forms, into the following natural 

 families : — 



A Table, showing the Sections and Families of the Echinoidea. 



Order. 



Order ECHINOIDEA. -i 



Sections. 



Families. 



c, . fClDARID^. 



Section A. Hemicidarid.e. 



Echmoidea enaocyclica. J r),.„p.„,T, „ 

 Vent within the genital plates, 1 Echinid^ ' 

 always opposite the mouth, |^ Saleniad^. 



TEchinoconid^. 

 Collyritid^. 

 Section B. Echinonid^. 



Echinoidea exocyclica. J Echinanthid^. 

 Vent without the genital plates, | Echinolamfid^. 

 never opposite the mouth. Clypeastertd^. 

 Echinocorid^. 



SPATANGIDiE, 



