LIST OF PLATES xv 



PACING FACE 



PLATE VII 104-105 



FIG. 



I. Diagram of contiguous halves of an interambulacrum and an 

 ambulacrum of Plesiechinus ornatus, showing "localized 

 stages in development." The first-formed plates (at the 

 bottom of the figure), and the last-formed plates (at the top) 

 show "youthful " characters of ornament. The full species- 

 character is shown at the mid-zone. 



2-13. Morphogeny of some types of apical systems in the Echinoidea. 

 Fig. 2. Diademine (e.g. Hemtpedina). Fig. 3. Calycine 

 (Peltastes}. Fig. 4. Plesiechinus. Fig. 5. Pygaster. 

 Fig. 6. Holectypus. Fig. 7. Discoidea (the posterior 

 genital is not always perforated). Fig. 8. Clypeaster. 

 Fig. 9. Anorthopygus. Fig. 10. Spatangus (after Loven). 

 Fig. ii. Adult Spatangus (after Loven). Fig. 12. Conulus. 

 Fig. 13. Echinocorys. For further explanation, see p. 94. 



PLATE VIII 104-105 



PHYLOGERONTIC CHARACTERS 



Parkinsonia dorsetensis (much reduced). Bathonian, Burton 

 Bradstock, Dorset. The inner (early) whorls are strongly 

 ribbed. Gerontic laevigation obliterates ornament in the 

 outer whorls. 



2. Clavella longaeva. Eocene, Barton, Hants. The protoconch 

 is followed by one whorl showing " Fusid" cross-ornament. 

 The two next whorls are smoothed by gerontic laevigation, 

 the rest become secondarily spinous by excessive calcifica- 

 tion. (Based on Grabau's figure.) 



3. Clavella solanderi. Eocene, Barton, Hants. A type similar to 



C. longaeva, but failing to progress beyond laevigation. 



4. Phillipsia gemmulifera (after Woodward). Carboniferous Lime- 



stone. Showing gerontic features in simplification of the 

 glabella, multiple pygidium, and pustular ornament. 



