ARISTOTLE'S LANTERN AND PERIGNATHIC GIRDLE. 



191 



Text-figs. 217-220. — Morphology of radii in Echini. The corona consists of five part.s, each inchiding an amljulacrmn 

 and two half-interanibiilacra originating beneath an ocukxr (pp. 3.5, 4.3, 62, 86). 



217. Palaeechitius quadriserialis Wright. Lower Carboniferous. Adapted from Plate 30, fig. 3. X 2.5. Seen from 

 the exterior. 



218. Eucidaris tribtdoides (Lamarck). Bahamas. R. T. J. Coll., 754. X 2.7. Seen from the interior with that 

 portion of the lantern and muscles associated with this coronal element (compare text-figs. 210, 222). 



219. Centrecliinus selosus (Lcske). Florida. R. T. J. Coll., 790. X 1.9. Base of a coronal element and that ])or- 

 tion of the lantern and muscles that are associated with this element (compare text-figs. 211, yi. 184; 225, p. 193). 



220. The same. Face view of the two radial half-pyramids, etc., that arc associated with one coronal element . Letter- 

 ing: ip., interpyramidal muscles; r. p., right half of pyramid 2, and /. /;., left half of jiyramid 3. 



Letters of text-figs. 218-220 as in text-figs. 221-230 (p. 193). 



