PLATE XII P. 



Arachnoides placenta, f. 1 - Jf. 



1. Young specimen, from the actinal side, nat. size. 



2. Same from above, nat. size. 



3. Oilier specimen from the actinal side, | nat. size. 



4. Same, with part of the abactinal floor removed, | nat. size. 



Ecbinodiscus biforis, /. 5, 6, 



5. Seen from the actinal side, nat. size. 



6. Part of abactinal floor removed to show the internal structure, nat. size. 



