PLATE XXV 



ONTOGENY OF THE LOOP IN TEREBRATALIA OBSOLETA 

 DALL (PAGES 408, 409) 



FIGURE 1. Interior of dorsal valve in early cistelliform stage ; show- 

 ing median septum and dental sockets. X 25. 



FIGURE 2. Side view of septum of preceding. X 25. 



FIGURE 3. A little later stage ; showing growth of two transverse 

 expansions on septum. X 25. 



FIGURE 4. Side view of same. X 25. 



FIGURE 5. Beginning of platidiform stage ; showing groove on edge 

 of septum, with arched covering at posterior end, and growth of crural 

 processes. X 25. 



FIGURE 6. Side view of same. X 25. 



FIGURE 7. Side view of septum at a later stage, just before growth 

 of descending branches of loop ; showing form and position of ascending- 

 branches, or secondary loop, and septal characters. X 25. 



FIGURE 8. Platidiform stage ; showing union of descending branches 

 with septum. X 12. 



FIGURE 9. Beginning of ismeniform stage; showing growth of as- 

 cending branches. X 12. 



FIGURE 10. Early muhlfeldtiform stage ; showing holes developed in 

 ascending branches by resorption. X 12. 



FIGURE 11. Oblique view of septum, with ascending branches 

 broken off. X 12. 



FIGURE 12. Beginning of terebrataliform stage ; showing union of 

 ascending with descending branches. X 12. 



FIGURE 13. A little later stage ; showing resorption of broad de- 

 scending branches. X 7. 



FIGURE 14. Late adolescent stage ; showing connecting bands from 

 descending lamellae to septum. X 2|. 



FIGURE 15. Fully adult loop, with connecting bands. X 2^. 



