374 ECHINOZOA. 



series of Echinoids possesses both "regular" and "irregular" types, 

 and in this respect runs parallel with the later group of the Euechin- 

 oids. It is to be remembered, however, that the abnormal Echino- 

 cystites (Cystocidaris) exhibits an irregularity of structure which 

 differs considerably from that which characterises the "Irregular" 

 Euechinoids. 



The sub-order of the Cystocidaridce comprises only the aberrant 

 genus Echinocystites (= Cystocidaris, Zittel), in which the interambu- 



Fig. 245. — a, Apical disc of Palcechinus, enlarged (after Baily) ; b, Apical disc of Melonites 

 (after Meek and Worthen) : g, One of the genital plates ; 0, One of the ocular plates. 



lacral zones are composed each of several rows of plates, and the 

 anus is excentric. The two known species of the genus are found 

 in the Silurian rocks of Britain. 



The test in Echinocystites is spheroidal or ovoid, with narrow ambulacral 

 and broad interambulacral areas, the latter being formed by several 

 irregular rows of imbricated calcareous plates, carrying spines. The 

 mouth is on the under surface, central in position, with a well-developed 

 masticatory apparatus ; while the anus is excentric, but has the unique 

 position of being not widely remote from the apex in one of the inter- 

 ambulacral spaces. The madreporite is also quite peculiar in not being 

 at the apex of the shell, but in being placed excentrically in one of the 

 interambulacra. Another remarkable point is that the anus is closed by 

 a valvular pyramid of calcareous plates, a feature highly characteristic of 

 the Cystideans. 



The sub-order of the Bothriocidaridce includes the single genus 

 Bothriocidaris, in which the interambulacral areas are composed 

 each of a single row of plates, and the anus is placed in the centre 

 of the apical disc. The two known species of the genus have been 

 found in the Ordovician rocks of Russia (Esthonia). 



