ECHINOIDEA. 



217 



mouth the marginal plate of the posterior ambulacrum is wide and pro- 

 jecting and called the labrum (Fig. 163). The interambulacral plates 

 (or plate) next to the labrum is called the sternum (st), and the next the 

 episternum (ep). When, as happens in most Clypeastroids, the five 

 interambulacral peristomial plates are cut off from the rest of the inter- 

 ambulacral by the widening out of the peristomial end of the ambulacral 

 plates, the interambulacra are said to be interrupted. 



In the Scutellidae a family of Clypeastroids, the margin of the flattened 

 test may be lobed, or incised at the edge. These incisions are some- 

 times included in the shell, in which they constitute perforations called 

 lunulae. The incisions which are formed during growth, the margin being 

 at first entire, may be very numerous (Rotula), or few. The lunulae are 

 always radial, excepting one which occurs in the posterior interradius. 



FIG. 157. Scwtella subrotundata Lamk. Miocene, Bordeaux (after Zittel), a from below ; 

 b from above ; c section. Natural size. 



The apical system of the Exocyclica'is much modified. There 

 is a frequent tendency for the genital opening of the posterior 

 interradius to disappear, and in many Spatangoids the posterior 

 basal plate is also absent. The anal area is occupied either by 

 an extension of the right anterior basal with its madreporitic 

 pores (ethmophract condition, Holectypus], or the central is said 

 to persist and to fuse with the right anterior basal and posterior 

 basal, the plate so formed being perforated by madreporitic 

 pores (ethmolysian condition, many Spatangoids, Fig. 160), or 



