ECHINODERMA TA—ECHINOIDEA . 



581 



fTs^p- d f - h 



Fig. 1915. a-c, Archceocidaris wortheni : a, restoration of ventral surface {a, b, 

 ambulacral ; c-f, interambulacral plates) ; b, ideal reconstruction of plates resorbed in 

 fig. a, showing a single initial column of interambulacral plates ; c, plate with primary 

 and secondary spines ; d, Lepidocidaris squa?nosa^ an interambulacrum flanked by 

 ambulacra on each side, showing 5 columns of former at ventral surface and appear- 

 ance of 6th, 7th and 8th column above a heptagonal plate {H)\ <?, Rhoechinus gracilis^ 

 a partial internal mold showing order of appearance of columns of interambulacral plates, 

 X l/^ > /> ^- elegans^ ambulacrum enlarged ; g-j^ Oligcporus dance : g, interambula- 

 crum with part of amb. bordering, showing order of appearance of plates as shown by 

 numerals, X M ; ^^ ambulacral area, X i^> showing two partial additional columns of 

 plates ; i, amb. plates separated by silicification and spines, X 4- (^^^ after Jackson. ) 



five columns of interambulacrals at ventral border, increasing to 

 eight; spines slender, smooth or finely striated, of circular section. 

 Lower Burlington of Iowa. 



