PLATE XXV 



ONTOGENY OF THE LOOP IN TEREBRATALIA OBSOLETA 

 Dall (Pages 408, 400) 



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 platidifonn stage ; showing groove on edge 

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

 processes. X 25. 



Figure G. — 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 sejital characters. X 25. 



Figure 8. — Platidifonn 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 lamella) to septum. X 2|. 



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



