PLATE 11 



Plectorthis fissicosta (Hall) 

 1, 3, 5, 15.- — Dorsal, lateral, ventral, and posterior views of 

 exterior. 

 Ordovician (Maysville), Cincinnati, Ohio. Cat. 

 No. S306. X 1.5. 



Plectorthis jamesi (Hall) 

 2. — Ventral interior, showing dental plates and muscula- 

 ture. Cf. fig. 14. 

 Ordovician (Corryville), Cincinnati, Ohio. Cat. 

 No. S 316. xl.5. 



Plectorthis plicatella (Hall) 

 4. — Dorsal interior. The incurving brachiophore plates 

 which unite with the valve under the cardinal process 

 are clearly visible. Cardinal process like that of 

 Hebertella, see fig. 23. 

 9. — Ventral interior, showing a delicate dental plate. 



Ordovician (Maysville), Cincinnati, Ohio. Cat. 

 No. S298. X 1.5. 



Doleroides gibbosus (Billings) 

 6. — Ventral interior, to show characteristic musculature. 

 Cf. fig. 24. 

 Ordovician (Decorah sh.), Minn. Cat. No. S 537. 

 X 1.5. 

 16, 18. — Dorsal and ventral views of an old individual, 

 showing normal wide sulcus. 

 Ordovician (Black River), Fountain, Minn. Cat. 

 No. S 508. xl.5. 



Doleroides cf. gibbosus (Billings) 

 7, 8, 11. — Dorsal, ventral, and posterior views of exterior 

 of a well preserved individual, showing pionodemoid 

 outline. This particular specimen has a narrower 

 fold and sulcus than is usual in the species. 

 Ordovician (Black River), Chatfield, Minn. Cat. 

 No. S 507. xl.5. 



Doleroides pervetus (Conrad) 



10, 13. — Dorsal and ventral views of exterior. 



Ordovician (Black River), Lanesboro, Minn. Cat. 

 No. S 498. xl.5. 



Figs. Doleroides pervetus (Conrad) — Cont. 



12. — Dorsal interior, showing cardinalia. Brachiophores 

 supported by convergent plates as in Plectorthis and 

 Hebertella. 

 Ordovician (Black River), Fountain, Minn. Cat. 

 No. S 499. X 2. 



Hebertella occidentalis sinuata (Hall) 



14. — Young individual. Note resemblance to Plectorthis, 

 figs. 2 and 9. x 2. 



17,23. — Enlarged views, x 1.75, of cardinalia of an old 

 shell, showing shaft and compressed myophore of car- 

 dinal process, brachiophores and their supporting 

 plates. 



19,20,22,25. — Lateral, ventral, dorsal, and posterior 

 views of a large individual. Note convexo-concave 

 profile. X 1.5. 



24. — Ventral interior, showing muscle area with its double- 

 track adductor impression and subcrescentic diductor 

 scars. Adjuster impressions borne, when visible, on 

 base of dental plates. Notice callus deposit at back 

 end near pedicle callist; in some specimens this 

 almost completely obliterates the posterior portion of 

 the muscular field, x 2. 



26. — Adult dorsal interior, showing cardinalia. Shaft of 

 cardinal process simple, with compressed myophore. 

 Brachiophores supported by converging plates which 

 unite under the cardinal process; sockets defined by 

 fulcral plates. This is different from the usual type 

 in the Orthidx, in which the brachiophores are simple 

 rods or blades supported only by a swelling of adventi- 

 tious shell. X 2. 

 Ordovician (Maysville), Cincinnati, Ohio. Cat. 

 Nos. S388 (figs. 19, 20, 22, 25), S 397 (figs. 14, 

 24), and S451 (figs. 17, 23, 26). 



Hebertella frankfortensis Foerste 

 21. — -Ventral interior, showing bipartite adductor track. 

 Cf. fig. 24. 

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

 S440. x2. 



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