PLATE 6 



Figs. Ptychopleurella bouchardi (Davidson) 



1, 33. — Ventral interiors, showing musculature, pedicle cal- 



list, and median ridge. 

 3, 4. — Ventral and dorsal exteriors, showing prominent 



costje and lamellose exterior. 

 6, 32. — Dorsal interiors, showing cardinal process, brachio- 

 phores, and adductor impressions. 

 Silurian (Gotlandian), Gotland, Sweden. Cat. No. 

 S285. Figs. 1, 3, 4, 6 x2; figs. 32, 33 x 3. 



Ptychopleurella matapedia Schuchert and Cooper, n. sp. 



2, 5. — Dorsal and ventral exteriors of a silicified specimen, 

 showing strongly lamellose surface. 



High Silurian or basal Devonian, Upsalquitch road, 

 Matapedia, Quebec. Holotype. Cat. No. S287. x 2. 



Ptychopleurella lamellosa (Twenhofel) 



9. — Posterior view of holotype, showing slitlike delthyrium 

 produced by growth of lateral plates. 

 Ordovician (Ellis Bay), Ellis Bay, Anticosti. Cat. 

 No. 10411 Y. P. M.' X 5. 



Archseorthis electra (Billings) 



7. — Ventral exterior, x 2. 



8. — Ventral interior, showing callus extending forward 

 from muscular area. 

 16. — Dorsal interior, showing low median ridge, nearly ob- 

 solete cardinal process, and brachiophores. x 4. 

 Ordovician (Canadian), Levis, Quebec. Cat. No. 

 740, Nat. Mus. Canada. All silicified. 



Deltatreta typica Ulrich MS. 



10, 19. — Dorsal and ventral exteriors, x 1.5, showing costel- 



late exterior and hollow costellas. 

 14. — Portion of a silicified ventral valve, x 3, showing del- 

 tidium and large, subapical foramen. Faint radial 

 lines also visible on interarea. 

 30. — Silicified dorsal interior, x 2.25, with a ventral valve 

 lying in it. Primitive nature of brachiophores and 

 their lateral supports like those of Vellamo plainly 

 shown, also simple cardinal process and chilidial plates 

 projecting posteriorly from notothyrial margin. The 

 ventral interior shows well the pedicle callist. 

 Ordovician (up. Canadian, ca. 1000 feet beneath 

 top of Arbuckle Is. (formation name not yet given) ) , 

 near Berwyn, Okla. (U. S. G. S. loc. 191K). U. S. 

 G. S. colls., U. S. Nat. Mus. 



Deltatreta sp. 



15. — Ventral interior, showing prominent dental plates and 



pseudospondylium, the latter produced by a deposit of 



adventitious shell beneath the muscles. Cf. ventral 



interior in fig. 30; cf. also fig. 13. 



Ordovician (2d Ceratopea zone), "inlier," sect. 25, 



T 6 N, R 14 W, N. E. of Wichita Mts., Okla. 



U. S. Nat. Mus. X 2. 



Figs. Deltatreta dice (Walcott) 



II. — Dorsal interior, showing primitive cardinalia. 



13. — Ventral interior, showing pseudospondylium. Cf. fig. 

 15. 

 Drift (probably Ordovician (Canadian)), St. Albans, 

 Vt. Impressions from type specimens, U. S. Nat. 

 Mus. Cat. No. 52248. x 1.5. 



Glyptorthis cf. bellarugosa (Conrad) 



1 2. — Dorsal exterior, showing lamellose surface. 



Ordovician ("Simpson"), Criner Hills, Okla. Cat. 

 No. S 251. x2. 



Glyptorthis insculpta (Hall) 



17, 20, 21. — Ventral, dorsal, and lateral views of exterior. 



Lateral profile biconvex. Cf. same view of Hebert- 

 ella, pi. 11, fig. 19. 



1 8, 26. — Ventral interiors, showing "Orihis" pattern to 



perfection. Note muscle area; central adductor track 

 straight, diductor tracks elongate tear-shaped; and 

 adjustor scars visible at base of dental plates. Ex- 

 tending forward from the adductor track is a low sep- 

 tum which divides the closely apposed pallial sinuses 

 originating at the front ends of the diductor impres- 

 sions. Lateral spaces occupied by subreniform ovarian 

 impressions. Elevated lines in these impressions con- 

 sidered to be muscle attachments for ovarian bodies. 

 29. — Dorsal interior, showing an old shell heavily ridged 

 by pallial markings. Note adductor scars. The car- 

 dinalia of Glyftorthis are of the "Orthii" type, dif- 

 fering markedly from those of Hebertella. See pi. 11, 

 figs. 24, 26. 



Glyftorthis has been usually considered to be a sub- 

 genus of Hebertella, but its lenticular profile and inter- 

 nal structure relate it rather XoHesferorthis and Orthis. 



Ordovician (Richmond), Oxford, Ohio. Cat. No. 



S242. X 1.5. 



Orthostrophia strophomenoides (Hall) 



22. — Dorsal internal mold, showing muscle area and pallial 



markings. 

 25. — Impression from the above, showing elevated muscle 

 area. 

 Devonian (New Scotland), near Clarksville, N. Y. 

 Cat. No. S 320. X 2. 

 27, 28. — Ventral and dorsal exteriors, showing characteris- 

 tic ornamentation. 

 Devonian (Birdsong), Henry Co., Tenn. Cat. No. 

 9702 Y. P. M. xl. 



Orthostrophia aff. O. strophomenoides (Hall) 



24. — Ventral internal mold, showing small muscle area and 

 pallial markings. 

 Devonian (Hunton), near Crusher, Okla. Cat. No. 

 S322. xl.5. 



Orthostrophia dartae Schuchert and Cooper, n. sp. 



23. — Ventral internal mold of paratype, showing small mus- 

 cle area. 

 3 1 . — Dorsal exterior of holotype, showing ornamentation. 



Silurian (Bouleaux), Port Daniel, Quebec. Cat. 



No. S 1985. X 1.5. 



206 



