416 STRATIGRAPHICAL DISTRIBUTION OF 



importance, although when taken in connexion with others it is valuable in the definition 

 of families. 



The position of the anal opening affords a good primary character ; in one section the 

 vent opens within the centre of the apical disc, surrounded by the genital and ocular 

 plates ; in another section the vent opens without the apical disc, and is external to, and 

 at a greater or less distance from, the genital and ocular plates ; these two sections may 

 be thus defined. 



ECHINOIDEA ENDOCYCLICA. 



A. Test circular, spheroidal, more or less depressed, rarely oblong ; mouth central 

 and basal; vent in the centre of the upper surface directly opposite the mouth, and 

 surrounded by five perforated genital and 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 external to the circle of genital 

 and ocular plates, never opposite the mouth, situated in different positions in relation to 

 that opening : four of the genital plates are generally perforated ; the fifth is in general 

 imperforate. Mouth sometimes armed with jaws, but oftener edentulous. Jaws 

 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 

 semitse, the size and form of the elements of the apical disc, and the position of the anus, 

 afford collectively good characters for defining 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 outline 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 prominence of the base ; the size of the tubercles ; the presence or 

 absence, the size and arrangement of the granides 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 ; the length of the spines ; the form of their 

 stems ; the character of the sculpture thereon ; the size of the head, the prominence and 



