CLASSIFICATION. 29 



value to the position of the anal opening ; and hence it forms the basis of the subdivision 

 of this order into two sections, which are thus defined : 



Echinoidea endocyclica. 



Test circular, spheroidal, more or less depressed, rarely oblong ; mouth in the centre 

 of the base. Vent in the centre of the upper surface, directly opposite to the 

 mouth, surrounded by five perforated genital plates, and having external to them 

 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 ex-central. Vent external to the circle 

 of genital and ocular plates, never opposite the mouth, and situated in different 

 positions in relation to that opening : four of the genital plates are generally 

 perforated. The mouth is sometimes armed with five jaws, oftener it is 

 edentulous. The elements of the lantern are disposed in a more or less horizontal 

 direction. 



The structure of the ambulacral areas, and poriferous zones, afford good characters 

 of secondary importance for grouping the genera into natural families, especially when 

 taken in connection with the position of the vent, which varies in its relation to that of 

 the mouth-opening in different families. 



The form, number, and arrangement of the tubercles, and the spines that are jointed 

 with them ; the miliary granulation ; the bands of microscopic granules forming the fas- 

 cioles, having permanent positions on the test ; also the size and number of the 

 elements of the apical disc, and the position of the vent, afford collectively good characters 

 for defining the genera. 



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

 tubercles on each of them ; the arrangement of the pores, their proximity or remoteness 

 from each other in the zones ; the general outline of the test, which has only certain 

 limits of variation ; the form of the areolas; the presence, absence, size, and distribution, 

 of the granules forming the scrobicular circle ; the completeness or incompleteness of the 

 same ; the length of the spines, their form and sculpture, are points affording good specific 

 characters, as they are persistent details that are developed on every considerable fragment 

 of the test and spines of the Echinoideas. 



