PLATE 8 



Figs. Gonambonites planus Pander emend. Pahlen 

 1. — Ventral interior, showing spondylium and its acces- 

 sory septa. Marginal thiclcening visible also. 



2, 3, 20. — Dorsal, ventral, and posterior views of a small 

 individual, showing prominent process extending from 

 deltidium. Fig. 20 shows deltidium somewhat 

 crushed in. 



4, 7. — Dorsal interiors, showing cardinalia and chilidial 

 plates. Brachiophores to be seen protruding from 

 lateral adventitious thickening just antero-laterally of 

 notothyrium. 



5,21,26. — Posterior, ventral, and dorsal views of a large 

 shell, showing characteristic ornamentation. 

 Ordovician (Canadian, Walchow, Bj), Wassilkowo, 

 near Lake Ladoga, Russia. Cat. No. S 333. x 1.5. 



25. — Ventral interior of a large specimen. 



Same locality as above. Shaler Mem. Exped. Colls., 

 Mus. Comp. Zool., Harvard Coll. x 1.5. 



Estlandia marginata (Pahlen) 

 6. — Dorsal interior, showing cardinalia and median ridge. 

 Brachiophores may be seen in postero-lateral portions 

 of shell just outside adventitious thickenings anterior 

 and laterally of notothyrium. 

 8. — Ventral interior, showing spondylium. Foramen in 

 deltidium closed by a plug of shell substance. 

 Ordovician (Kuckers, Brandschiefer), Baron Toll's 

 estate, near Jewe, Estonia. Cat. No. S 327. x 1.5. 



9. — A fine dorsal interior, showing cardinalia. Notice 

 brachiophores outside anchor-shaped adventitious 

 deposits. 



Middle Ordovician (Cgo), Kohtla, Estonia. Cat. 



No. S 2106. X 1.5. Presented by A. Opik. 



Hemipronites tumidus Pander 

 10. — Ventral interior with deltidium missing, showing 

 median septum dividing two pallial trunks which in 

 turn surround inside margins of subreniform ovarian 

 impressions. Cf. Glyftorthis insculfta, pi. 6, figs. 18, 

 26, and Orthis, pi. 2, fig. 18. It will be noted that 

 the spondylium is the homologue of the dental plates 

 and floor of the deltidial cavity. The septum is the 

 homologue of the small ridge dividing the pallial 

 sinuses in Orthis s.s.; the sinuses are situated in 

 precisely the same place and bound similar ovarian 

 impressions. 

 Ordovician (Walchow, Bj), Popovka, near Lenin- 

 grad, Russia. Cat. No. S3 69. x 1.5. 



Figs. Hemipronites tumidus Pander — Cont. 



11. — Dorsal interior, showing cardinalia. Cardinal process 

 simple as in Orthis s. s., and brachiophores rodlike 

 plates supported by adventitious tissue which, in this 

 instance, spreads laterally to form an anchor-shaped 

 mass with the median ridge. 



14, 15. — Posterior and ventral exteriors, showing ornamen- 

 tation. 

 Ordovician (Walchow, Bj), Popovka, near Lenin- 

 grad, Russia. Cat. No. S 369. x 1.5. 



Hemipronites cf. maximus Pander 

 12,13. — Ventral and dorsal exteriors, showing fine costellae. 

 Ordovician (Canadian, Walchow), Popovka, near 

 Leningrad, Russia. Cat. No. S 365. x 1. 



Marionella typa Bancroft 

 16. — Dorsal internal mold, showing dinorthid character of 



cardinalia. 

 17. — Muscle-scars and pallial markings of ventral valve. 

 18. — Impression showing external sculpture. 



Ordovician (upper Longvillian), Horderley District, 

 East Shropshire, England. Cat. No. S2107 (fig. 

 18), and S2108. x 1.5. 



Multicostella platys (Billings) 



19, 23. — Ventral and dorsal exteriors, showing ornamenta- 

 tion of this biconvex dinorthid. 

 Ordovician (Chazy), Speer's Ferry, Virginia. Cat. 

 No. S92. X 1.5. 



Multicostella saffordi (Hall and Clarke) 

 22. — Ventral interior, showing musculature, which is simi- 

 lar to that of Valcourea. 

 27. — Dorsal interior, with crenulated dinorthid cardinal 

 process plainly visible. Brachiophores also similar to 

 those of Valcourea. 



Multicostella was common in Chazy time, but evi- 

 dently did not survive this stage. A deltidium or 

 chilidium is unknown in this dinorthid stock. 

 Ordovician (Chazy), Washburn, Tenn. Cat. No. 

 S786. X 1.5. 



Clinambon anomalus (Schlotheim) 



24, 28. — Posterior and lateral views, showing unusual de- 

 velopment of chilidium, and posterior portion of 

 dorsal valve. 

 Ordovician (Mohawkian, Kegel, D,), Redder, Es- 

 tonia. Cat. No. S 337. x 1.5. 



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