54 



ROBERT TRACY JACKSON ON ECHINI. 



the two peristomal rows. In young Goniocidaris (Plate 2, figs. 1-3) the ambulacral plates are 

 also high, hexagonal, and there are few tube-feet, about ten to each area. From this condi- 

 tion, which I would call primitive, increase of tube-feet or increase of locomotive power, is 

 attained on three independent lines of development. First and simplest, by producing low 

 plates with pore-pairs uniserial, so that many plates are contained in the vertical extent of the 

 area, as in Archaeocidaris (Plate 9, fig. 6), Eucidaris (text-fig. 4), and Palaeechinus (Plate 30, 

 fig. 3). A second method of getting tube-feet is by increasing the number of columns of ambu- 

 lacral plates from two to a greater number, but always by even numbers and always by simple, 

 never compound plates (text-figs. 10-21). The increase is from two to four, six, eight, ten, 

 twelve, sixteen, or twenty columns of plates in an area, which is the most at present known. 

 This method is known in the Palaeozoic Echini only, and is especially developed in the Palae- 

 echinidae and Lepidesthidae. It is, however, taken up as a method in other types, as Echinocy- 

 'stites. The third method of attaining increase of tube-feet is by building compound plates, 



4 



formed by the coalescence of originally simple plates, Centrechinus (text-figs. 92, 94). The 



I 



9 



10 



TEXT-FIGS. 2-14. Character of the ambulacrum in representative Echini; left half represented. The horizontal 

 dotted line is on the plane of the mid-zone. 



2. Bothriocidaris archaica sp. nov. Ordovician. From Plate 1, fig. 1. 



3. Goniocidaris canaliculate A. Agassiz. Young. From Plate 2, fig. 2. 



4. Eucidaris tribuloides (Lamarck). Bahamas. 



5. Strongylocenlrolus drobachiensis (O. F. Muller). Young. From Plate 3, fig. 11. 

 5a. The same. Adult. York Harbor, Maine. 



6. Micrasler cor-anguineum (Lamarck). Cretaceous, England. 



7. Echinarachnius parma (Lamarck). Eastport, Maine. 



8. Metalia pectoralis (Lamarck). Bahamas. Showing plates of two areas. 



9. Palaeechinus elegans M'Coy. Lower Carboniferous. From Plate 31, fig. 1. 



10. Maccoya burlingtonensis (Meek and Worthen). Lower Carboniferous. From Plate 33, fig. 2. 



11. Lovenechinus missouriensis (Jackson). Lower Carboniferous. From Plate 43, fig. 3. 



12. Oligoporus danae Meek and Worthen. Lower Carboniferous. From Plate 50, fig. 7. 



13. Melonechinus multiporus (Norwood and Owen). Lower Carboniferous. From Plate 56, fig. 4. 



14. Lepidesthes colletti White. Lower Carboniferous, From Plate 70, fig. 3. 



