PLATE XII. 



1. Calcareous reticulation of side plate of food furrow of middle of arm. 



2. Another side plate from the same region. 



3. 4, 5. Different types of side plates of the same groove. 



6. Side plate from groove near end of arm. 



7. Covering plate of end of arm. 



8. Oblique view of fifth joint from end of arm. 



9. Profile of a young arm joint near end of another arm (similar to Fig. 8). 



10. Calcareous floor separating the axial canal of the ai-m from the food groove of a joint about in the 



stage of Fig. 9. 



11. A young arm joint, the penultimate. 



12. End view of same, showing axial canal. 



13. Terminal arm joint. 



14. Extremity of arm. 



15. 16. Articulations of young terminal arm joints. 

 17. Syzygy of young arm joint. 



