THE ECHINOIDEA 301 



through the sutures between the plates. Near the aboral pole the 

 ambulacra are narrower and pores occur in the plates. Genus 

 Palaeodiscus, Salter, from the Silurian of Ludlow. It is the most 

 primitive of known Echinoids and has been frequently assigned to the 

 Stelleroida. The main radial water-vascular vessels appear, however, 

 to have passed along the inside of the test instead of below or outside 

 the ambulacral plates, ae in Stelleroids (Salter, 42 on p. 280, ante; Neu- 

 mayr, 64 ; Gregory, 36). 



FAMILY 2. ECHINQCYSTIDAE. Cystocidaroida, in which the ambulacra 

 consist of narrow plates, each perforated by a pore-pair. The pore-pairs 

 are biserial ; most of the plates are low primaries, but demi-plates also 

 occur. Genus Echinocystis, Wyv. Thorns, (non Hall), Silurian; one of 

 the most remarkable of known Echinoids. It has no apical system of 

 plates, and the anus and madreporite both open independently in the 

 posterior interradius. The genus is therefore sometimes described as 

 exocyclic, but it is really acyclic (Gregory, 36). 



ORDER 3. Cidaroida. 



Echinoidea Regularia Endobranchiata, in which the peristome is 

 central ; the periproct is central on the aboral surface of the body, and is 

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

 of two vertical series of simple narrow plates, some of which may be 

 demi-plates. The interambulacral plates are unituberculate, bearing large 

 spines. There is a dental apparatus. 



In the Devonian system the Echinoids are scarce, but their characters in- 

 dicate a marked advance upon the Silurian species in the strength of the tests, 

 owing to the greater thickness 

 and regularity of the plates. 

 Two main lines of differen- 

 tiation are apparent. In the 

 first the increase takes place 

 in the interambulacral plates, 

 in the second the ambulacral 

 plates become more import- 

 ant. The former is the order 

 Cidaroida, the latter is the 

 order Melonitoida. 



There are four families 

 of Cidaroida, of which three 

 are extinct. The most typical 

 genus is Cidaris ; the earliest D of ^^ of plates near peri8tome 



and most primitive IS Lepido- in Lepidocentrus. Devonian, Germany. 



centrus. 



FAMILY 1. LEPIDOCENTRIDAE. Cidaroida with ambulacral pore-pairs 

 in a single series. Interambulacral plates in more than two vertical rows. 

 Test flexible, owing to imbrication of the plates. No interambulacral 

 plates pass on to the peristomial membrane. This family is represented 

 by four Palaeozoic genera Lepidocentrus, Miiller, Devonian ; Lepidechinus, 



