40 Mr. W. K. Spencer. On the Structure and [Mar. 22, 



throughout most of their depth. An examination of the series of 

 photographs as also the wax models show that no pores are present 

 leading into the interior of the test. These plates were on an average 

 13 mm. in thickness, the plates covering the mouth are merely 

 enlarged side covering plates. They occupy the angles of those 

 inter-radii which border the mouth region. These inter-radii are not 



\J 



m.ai. 



s.cp. 



TEXT FIGS. 9, 10, 11, 12. Progressive views of reconstructed wax model of 

 Lepidodiscus (Agelacrinus) pileus ; /, II, III, IV, V = ambulacra ; 

 s.cp., side-covering plates ; m.ai., mouth plate of anal inter-radius ; 

 m.cp., mouth corner plate ; f.p., flooring plates. By error in the figures, 

 the ambulacra have been numbered V, I, II, III, IV instead of 

 I, II, III, IV, V. 



FIG. 9. Outermost view. 



five in number but three, namely those between II and III, III and 

 IV, IV and V. This is because a secondary triradiate symmetry has 

 been imposed upon the primitive pentamerous symmetry of this genus, 

 vide infra. 



Two of the mouth-covering plates are of equal size. They are 

 smaller than the third, which occupies the angle of the anal inter- 



The ambulacral groove is floored by a series of single oblong plates, 

 which stretch across the groove, and show no trace of median suture or 

 pore, figs. 10, 11. They do not correspond in number to the side covering 

 plates, but are somewhat larger, figs. 10,11. Jaekel (8, p. 1 7) has the same 



