PLATE 1 



YiGs. Bohemiella romingeri (Barrande) 



1^ 18. — Wax replicas of dorsal interior, showing cardinalia 

 which are orthoid in aspect. Cf. pi. 4, fig. 17. Note 

 differences from cardinalia of BllUngsella and Eoor- 

 ihis, figs. 13 and 28. 

 3. — Internal mold of small ventral valve, showing diver- 

 gent pallia! trunks. 

 22. — Wax replica of dorsal exterior. Cf. fig. 6. 



Mid.Cambrian,Skrej, Bohemia. Cat.No. S12. x 2. 



5. — Dorsal exterior, showing also ventral valve in place. 

 Deltidium lacking and profile piano- or concavo- 

 convex. 

 1 1 . — Dorsal interior, showing remarkable growth of adven- 

 titious shell over brachiophores. Cf. pi. 5, figs. 1 7, 24. 

 Wax impressions of molds. Mid. Cambrian, Skrej, 

 Bohemia. Originals U. S. Nat. Mus. Cat. No. 

 52267. x2. 



Orusia atava (Matthew) 

 2. — Internal mold, showing slotlike molds of brachiophore 

 plates. Erroneously referred to Eoorthis by Walcott. 

 After Walcott, Camb. Brach., pi. 95, fig. 7b. x 0.5. 



Orusia lenticularis (Wahlenberg) 



7. — Ventral internal mold, showing dental plates. 



9. — Dorsal internal mold, showing slotlike molds of brach- 

 iophore supporting plates, probably very similar to 

 those of Finkelnburgia. 



After Walcott, Camb. Brach., pi. 98, figs. 2d, 2f'. 



Reduced. 



Oligomys exporrectus (Linnarsson) 

 4, 15. — Ventral internal mold, showing impression of small 

 delthyrial cavity and muscle tracks. 

 Mid. Cambrian (Paradoxides zone), Westrogothia, 

 Lovened, Sweden. Cat. No. S 16. Fig. 4 x 2.65 ; 

 fig. 15x6. 

 8. — Dorsal interior, showing cardinalia. Cf. Billingsella, 

 fig. 13. The brachiophores of Oligomys, when com- 

 plete, are similar to those of OrtAis s. s. Replica of 

 specimen figured by Walcott, Camb. Brach., pi. 88, 

 fig. 1 1. 

 17. — Dorsal internal mold, showing pallial marks. After 

 Walcott, Camb. Brach., pi. 88, fig. Ik. Reduced. 

 Mid. Cambrian (Agnoi/us Itevigatus zone), Gud- 

 heim, 12.5 miles SSE of Skara, Sweden. Cat. No. 

 52249 U. S. Nat. Mus. 



Billingsella cf. pepina (Hall) 

 6^ 19_ — Dors.il and ventr.il exteriors, showing ornamenta- 

 tion. 

 1 3. — Dorsal interior, showing cardinalia. Note short brach- 

 iophores. Cf. Eoorthis, fig. 28. 

 21^25. — Ventral interiors, showing perforate deltidium. 

 Fig. 25 shows thickening of median portion of mus- 

 cular area. Note the large teeth. The deltidium is 

 an arch as in other orthoids. Cf. pi. 7, fig. 16. 

 Up. Cambrian, Grand Teton, S. of Muskrat Lake, 

 Teton Creek, Idaho-Wyoming state line. Cat. No. 

 S2220. x2. 



Figs. Billingsella lindstroemi (Linnarsson) 



10. — Ventral internal mold, showing divergent pallial trunks 

 and reniform ovarian impressions. Cf. Orthis, pi. 2, 

 fig. 18. After Walcott, Camb. Brach., pi. 87, fig. 6e. 

 Reduced. 

 Mid. Cambrian {Paradoxides jorchhammeri zone), 

 Alunbruk, Oeland I., Sweden. 



27. — Ventral internal mold, showing pallial markings and 



adherent fragments of shell. A thin section of the 



shell substance (see pi. 29, fig. 12) shows it to be 



fibrous, and fibres are visible with a hand lens also. 



Mid. Cambrian (Paradoxides zone), Westrogothia, 



Lovened, Sweden. Cat. No. S 20. x 2. 



Protorthis billingsi (Hartt) 

 12. — Ventral internal mold, showing free spondylium. 



Cambrian (Acadian), Seeley St., St. John, N. B. 

 Cat. No. S94. x2.65. 



14. — Ventral valve, showing free spondylium. 



After Hall and Clarke, Pal. N. Y., vol. 8, pt. 1, 

 1892, pi. 7A, fig. 16. Reduced. 



Nisusia festinata (Billings) 

 16. — Wax replica of exterior of a small ventral valve. 



Low. Cambrian, near Emigsville, York Co., Penn. 

 Cat. No. S 9. X 2. 



20. — Dorsal internal mold, showing musculature. Cardi- 

 nalia similar to those of Billingsella but more primi- 

 tive. After Walcott, Camb. Brach., pi. 100, fig. Ig. 

 Reduced. 

 Low. Cambrian, near Emigsville, Penn. 



Eoorthis remnicha (Winchell) 

 23. — Dorsal interior, showing cardinalia. A low cardinal 

 process or ridge has risen between the seats of muscle 

 attachment on the notothyrial platform. 

 Up. Cambrian, S. side Gallatin Valley, Mont. 

 U. S. Nat. Mus. X 2. 



Eoorthis cf. remnicha (Winchell) 

 26. — Ventral interior, showing thickening on floor of del- 

 thyrial cavity. This type of structure is called a 

 pseudospondylium, but the resemblance to a spon- 

 dylium is very remote in Billingsella and Eoorthis. 

 28. — Dorsal interior, showing flat, oblique brachiophores. 

 This specimen is deprived of a cardinal process, hence 

 the muscles were attached on the floor of the notothy- 

 rial cavity. 

 Up. Cambrian, Flat River, Mo. U. S. Nat. Mus. x 2. 



Jamesella cf. perpasta (Pompeckj) 

 24. — Ventral internal mold of a large specimen, showing 

 muscular marks. 

 Up. Cambrian, Skrej, Bohemia. Cat. No. S 1944. 

 x2. 



196 



