PLATE XI 

 ATREMATA (Page 243) 



Figure 1. — Dorsal valve of Iphidea lahradurica swantonensis Bil- 

 lings. X 3. 



Figure 2. — Ventral valve of young specimen, x 3. 



Cambrian, near Georgia, A'^ermont. 



Figure 3. — Apex of ventral valve of Glottidia pyramidala Stimp- 

 son. X 25. 



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

 minal protegulum. X 2.5. 



Recent, Beaufort, North Carolina. 



NEOTREMATA (Page 244) 



Figure 5. — Upper valve of nepioiiic Orbiculoidea minula Hall; repre- 

 senting protegulum, p, and Paterinu 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 

 Irpj-is Sowerby ; corresponding to neanic stage of Orbiculoidea minuta. 

 figure 0. X -'<■ 



Figure 9. — Ventral protegulum of same species similarly modified ; 

 agreeing with figure 7. X 25. 



FiGUHK 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 Acrothele suhsidua (after Linnarsson) ; 

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

 brian, Sweden. 



