THE ECHINOIDEA 303 



Fig. XVII. gives the aboral surface of Cidaris (Stereocidaris) subvesi- 

 culosa, from the Chalk, showing its primitive dicyclic apical system ; its 

 massive interambulacral plates separated by very narrow ambulacra, com- 

 posed only of low, simple primaries (Fig. XII. 1). The arrangement of 

 the peristomial plates in this genus is very important ; the peristomial 

 membrane is covered by loose plates which include representatives of 

 both the ambulacral and interambulacral series. There are no arched 

 processes over the ambulacra, the perignathic girdle consisting only of 

 interradial ridges (Fig. XIII. 1). The internal gills, 1 or Stewart's organs, 

 are well developed. 



FAMILY 4. DIPLOCIDARIDAE. Cidaroida with ambulacral pore-pairs 

 biserial. Interambulacral plates in two or more vertical series in each 



^M 



Cidaris (Stereocidaris) subvesiculosa, d'Orbigny ; from the Chalk. 



area. Peristomial plates as in Cidaridae. This family includes the 

 interesting genus Tetracidaris, Cotteau, which has four rows of plates in 

 each interambulacrum. This Palseechinoid character is associated with 

 a type of ambulacrum which, for the Cidaridae, is remarkably specialised. 

 A second genus is Diplocidaris, Desor. 



ORDER 4. Melonitoida. 



Echinoidea Eegularia Endobranchiata, in which the peristome is cen- 

 tral on the lower surface, and the periproct central on the upper surface, 

 surrounded by the apical system of plates. The ambulacra each consist of 

 two or more rows of simple plates, of which some or all may be demi- 

 plates, or klasma-plates. The interambulacral plates are covered by 

 granules bearing short, simple spines ; but occasional tubercles may occur. 



1 It should be remembered that the respiratory function of these organs is still 

 hypothetical. 



