302 PKOr. P. M. DUNCAN 8 EETISION OF THE 



or piece of plain test passing for some distance dorsally from the grooves, 

 and they are in relation to small tufts of external branchicB. 



Spheridia are either placed visibly upon short stalks in the ambulacra, 

 usually near the peristome, or they may be partly or entirely covered by 

 test • they vary in number and exact position, and are opalescent sphe- 

 roidal bodies. (See Loven, Etudes.) 



Interraditim. — One of the five areas vehich are placed between the ambulacra, 

 and reach from the basal plates of the apical system to the peristome 

 (coronal), and in some instances to the mouth {peristomiaT). 



The plates are more or less geometrical and in vertical rows, and 

 are united by transverse and oblique sutures witli those above and below, 

 and a median suture (or in some Palseozoic forms sutures) unites them also 

 at the side. Variable in shape and in namber ; when in more than two 

 vertical rows those nearest the ambulacro-interradial sutures are " adam- 

 hulacral." In the Euechinoidea the interradia correspond with basal 

 plates, and are numbered as they are, and receive the same orientation. 

 In the Exocyclica the posterior interradium, No. 5 (Loven), has the peri- 

 proct occupying some space at the median line or suture ; and in Spatan- 

 goida the posterior interradium is long, and often forms actinally a sternum, 

 preceded at and towards the peristome by a lahrum, forming the posterior 

 edge of the peristomial margin, followed or not by an epistermim. When 

 there is a large plate on either side of the median line of the sternum, the 

 test is amphisternous ; and when there is more or less of a zigzag and the 

 plates are not arranged symmetrically on either side of a median line, the 

 test is meridosternous (Lov^n, ' Pourtalesia'). The interradia may enter the 

 peristomial margin, each with two plates, or with onlj' one, as in Cassidulus 

 and in Spatangoids and Clypeastrid^. The interradia are usually con- 

 timcotts from apex to peristome, and in Oidaridse and in some Palseechi- 

 noidea they are continued beyond the peristome associated with ambulacral 

 plates to near the stoma. But discontinuity may happen by the enlarge- 

 ment of ambulacral plates blocking out the union of 'Co.q first and second peri- 

 stomial interradial plates, leaving a circle of ambulacral plates, as in Mellita 

 (Loyen, ' Etudes'). There is a want of symmetry and similarity between 

 the arrangements of the plates in t\i& postero-lateral pair of interradia (^os,. 

 1 & 4), and this is produced by the fusion of certain plates in the actinal 

 region of the right postero-lateral interradium No. 1 (Loven). The anterior 

 part of the interradium is zone b, and the posterior zone next to the postero- 

 lateral ambulacrum is zone a. (For descriptions of this want of symmetry, 

 ancient and modern, see Loven, ' Etudes.') 



Union of the second and third (2 & 3) plates of zone a, no such union 

 happening in the corresponding zone of interradium 4, is normal ketero- 

 nomy of interradium 1 ; union of the second plates of zone a and zone b in 

 interradium 1 is ancient heteronomy. Irregular heteronomy when the 

 platea 2 & 3 of zone b unite with plate 2 of zone a in interradium 1, the 

 plates 2 of both zones of interradium 4 also uniting {Palceostoma). 



The Peristome, more or less central and actinal in the Endocyclica, is 

 sunken or flush, decag9nal, pentagonal, with or without branchial incisions 

 or cuts ; the margin is composed of ambulacral and interradial plates, 



