EVOLUTION OF THE ECHINODERMS — FELL 



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Figure 9. — A, jaw of an asteroid, Persephonaster. B, jaw of the somasteroid Platasterias. 

 C, juvenile stage of the Cambro-Ordovician somasteroid Chinianaster, X 8. D. ventral 

 aspect of the jaw of juvenile Chinianaster. E, ventral aspect of the jaw in the Cambro- 

 Ordovician somasteroid Villebrunaster. F, Ampullaster, an Ordovician somasteroid 

 in which the tube-foot rested on a sunken shelf, with a pore for an interna! ampulla 

 bounded by the wings and adjacent adambulacral virgalia. 



Al, A2, A3, A4, first, second, third, and fourth ambulacral elements, Al forming the 

 jaw angle-plate. Fl, facet for interradial muscle of jaw. M, sieve-plate. Po, oral 

 pinnule. P3, P4, pinnules of third and fourth arm joints (in the case of Platasterias and 

 Villebrunaster, modified into fixed metapinnules). S, line of syzygy between A2 and A3. 

 T, terminal or T-shaped plate. TE, terminal pair of ambulacral ossicles. 



