PLATE XI 

 ATREMATA (Page 243) 



Figure 1. — Dorsal valve of Iphidea lahrudorica 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 pyramldala Stimp- 

 son. X 25. 



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

 minal pi'otegulura. X 25. 



Recent, Beaufort, North Carolina. 



NEOTREMATA (Page 244) 



Figure 5. — Upper valve of nepionic Orbicidoidea minuta Hall; repre- 

 senting proteguluni, 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 Orhiculoidea minuta. 

 figure G. 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. ivE, fig. 10, 1890.) 



Figure 12. — Lower valve of Acrnlhele suhsidua (after Linnarsson) ; 

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

 brian, Sweden. 



