PLATE 9 



FiGS. Dinorthis sweeneyi (Winchell) 



1. — Ventral interior, showing form of muscle impressions 

 and remnantal deltidium. x 1.5. 

 10, 14. — Lateral and dorsal views of exterior, showing con- 

 vexo-concave profile and costate exterior, x 1.5. 

 1 1 . — Dorsal interior, showing orthoid brachiophores and 

 crenulated myophore of cardinal process. In its later 

 stages of growth, the cardinal process may become 

 distinctly lobate, see figs. 3 and 20. x 2. 

 Ordovician (Decorah), St. Paul, Minn. Cat. Nos. 

 S686 and S 693. 



Dinorthis pectinella (Emmons) 

 2. — Ventral interior of a silicified specimen, showing in- 

 terior. X 1. 

 5. — Exterior, x 0.75, with its costae. 



Ordovician (Trenton), Curdsville, Ky. Cat. No. 

 S685. 



Dinorthis (Plaesiomys) subquadrata (Hall) 

 3. — Dorsal interior, showing cardinal process of an old 

 shell and lobation developed in late stages, x 2. 

 20. — Same, x 3, showing myophore and shaft, also orthoid 

 nature of brachiophores and swollen notothyrial plat- 

 form. Cf. pi. 10, fig. 25. 

 Ordovician (Richmond), Richmond, Ind. Cat. 

 No. S 728. 

 Dinorthis (Pionorthis) sola (Billings) 

 4^ 6^ 7^ 8. — Ventral, lateral, posterior, and dorsal views of 

 a large specimen, showing ornamentation and lentic- 

 ular outline of the species. 

 Ordovician (Richmond), Pt. Carleton, Anticosti. 

 Cat. No. S760. x 1.5. 

 9._Ventral internal mold, showing characteristic dinor- 

 thid muscle-scars. 

 Ordovician, cliff, W. side Vaureal Bay, Anticosti. 

 Cat. No. S 758. x 2. 

 Aff. Dinorthis (Pionorthis) carletona Twenhofel 

 13. — Ventral internal mold, showing muscle impressions. 

 Ordovician, Raven Nest, Anticosti. Cat. No. S 754. 

 X 1.5. 



FiGs. Dinorthis (Retrorsirostra) carleyi (Hall) 



21,23. — Ventral and dorsal exteriors, showing procline 

 ventral interarea. 

 Ordovician (Arnheim), Jefferson Co., Ind. Cat. 

 No. S 747. xl.5. 



Dinorthis (Retrorsirostra) carleyi insolens Foerste 

 22. — Ventral interior, showing teeth, crural fossetts, curv- 

 ing dental plates, and muscle impressions. Note 

 absence of pallial marks. 

 Ordovician (Waynesville) ? , Hanover, Ohio. Cat. 

 No. S 750. xl.5. 



Austinella sp. 



12. — Ventral interior, showing large teeth and outline of 

 muscle area. 

 Ordovician (Maquoketa), Spring Valley, Minn. 

 Cat. No. S 375. xl.5. 



Austinella whitfieldi (N. H. Winchell) 

 15. — Ventral internal mold, showing muscle-scars. These 

 and the pallial trunks diverging from the anterior 

 ends of the diductor impressions appear to relate the 

 genus most closely to Dinorthis. 

 16, 19. — Ventral and dorsal exteriors. 

 18. — Reverse side of fig. 15, dorsal internal mold, showing 

 adductor impressions. 



This genus has been referred to as a subgenus of 

 Plectorthis, but structurally it has nothing to do with 

 PUctorthis or the Plectorthidse. 

 Ordovician (Maquoketa), Spring Valley, Minn. Cat. 

 Nos. S 374 and S 376 (figs. 16, 19). x 1.5. 



Austinella scovillei (Miller) 

 22. — Ventral interior, showing teeth, crural fossettes, curv- 

 tinctly dinorthid in structure. 

 Ordovician (Richmond), near Lebanon, Ohio. Cat. 

 No. S 377. xl.5. 



212 



