C JE. Beecher — Development of Terehratalia ohsoleta^ Dall. 399 



PLATE III. 



Ontogeny of the loop in Terehratalia obsoleta Dall. 



Fig. 1. — Interior of dorsal valve in early cistelliform stage, showing median sep- 

 tum and dental sockets, x 35. 

 Fig. 2. — Side view of septum of preceding, x 25. 

 Fig. 3. — A little later stage, showing growth of two transverse expansions on 



septum. X 25. 

 Fig. 4. — Side view of same, x 25. 

 Fig. 5. — Beginning of platidiform stage, showing groove on edge of septum with 



arched covering at posterior end, and growth of crural processes, x 25. 

 Fig. 6. — Side view of same, x 25. 

 Fig. 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. 

 Fig. 8. — Platidiform stage, showing union of descending branches with septum. 



xl2. 

 Fig. 9. — Beginning of ismeniform stage, showing growth of ascending branches. 



xl2. 

 Fig. 10. — Early muhlfeldtiform stage, showing holes developed in ascending 



branches by resorption, x 12. 

 Fig. 11. — Oblique view of septum, with ascending branches broken off. x 12. 

 Fig. 12. — Beginning of terehrataliforin stage, showing union of ascending with 



descending branches, x 12. 

 Fig. 13. — A little later stage, showing resorption of broad descending branches. 



x7. 

 Fig. 14. — Late adolescent stage, showing connecting bands from descending 



lamellae to septum, x 23^. 

 Fig. 15. — Fully adult loop, with connecting bands, x 23^. 



