PLATE XI 

 ATREMATA (PAGE 243) 



FIGURE 1. Dorsal valve of Iphidea labradorica swantonensis Bil- 

 lings. X 3. 



FIGURE 2. Ventral valve of young specimen. X 3. 



Cambrian, near Georgia, Vermont. 



FIGURE 3. Apex of ventral valve of Glottidia pyramidata Stimp- 

 son. X 25. 



FIGURE 4. The same ; dorsal valve ; showing more distinctly ter- 

 minal protegulum. X 25. 



Recent, Beaufort, North Carolina. 



NEOTREMATA (PAGE 244) 



FIGURE 5. Upper valve of nepionic Orbiculoidea minuta Hall; repre- 

 senting protegulum, p, and Paterina stage. X 25. 



FIGURE 6. More advanced condition ; showing acquisition of dis- 

 cinoid characters. X 25. 



FIGURE 7. Lower valve of young specimen ; showing protegulum 

 and open pedicle-notch. X 25. 



Devonian, Marcellus Shale, Avon, New York. 



FIGURE 8. Accelerated, discinoid, dorsal protegulum of Discinisca 

 IcEvis Sowerby ; corresponding to neanic stage of Orbiculoidea minuta. 

 figure 6. X 25. 



FIGURE 9. Ventral protegulum of same species similarly modified ; 

 agreeing with figure 7. X 25. 



FIGURE 10. Lower valve of same species; showing sub-marginal 

 position of pedicle-opening. Natural size. 



Recent, Callao, Peru. 



FIGURE 11. Lower valve of Schizotreta tenuilamellata Hall; showing 

 centripetal tendency of pedicle-opening. Natural size. 



Niagara Group, Hamilton, Ontario. (Pal. N. Y. Extract from 

 vol. viii, pi. iv E, fig. 10, 1890.) 



FIGURE 12. Lower valve of Acroihele subsidua (after Linnarsson) ; 

 showing sub-central position of pedicle-opening. Natural size. Cam- 

 brian, Sweden. 



