PLATE 21 



Figs. Aff. Parmorthis visbyensis (Lindstrom) 



1, 6, 12. — Dorsal, ventral, and posterior views of a medium- 

 sized individual. 

 8. — Dorsal interior, showing adductor field and trilobed 

 cardinal process. The concavity of the dorsal valve 

 has caused a bending postero-dorsally of the cardinal 

 process in order to maintain the necessary leverage in 

 opening the valves. This makes the myophore visi- 

 ble on the exterior of the dorsal valves (see fig. 1 ) . 



1 1 . — Dorsal view of a larger shell. 



15. — Ventral internal mold, showing pallial trunks swing- 

 ing into apposition as they extend forward. This is 

 the usual condition in Parmorthis and is a significant 

 difference between these shells and Ddmanella s. s. 

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

 S895. x2. 



Parmorthis elegantula (Dalnian) 

 2. — Dorsal interior seen from rear, showing (on left 



brachiophore) an elongate process or crus (?). 

 3. — Posterior view of a dorsal interior, showing fulcral 

 plates and brachiophores. Parmorthis is the only dal- 

 manellid with corrugated dental sockets, teeth, and 

 fossettes. 

 13. — Dorsal interior, showing elongate-ovate adductor field 

 with its smaller anterior adductor scars. Fulcral plates 

 visible, and inside surface of brachiophores strength- 

 ened by adventitious material. 

 Silurian (Gotlandian), Gotland, Sweden. Cat. Nos. 

 S 886 (fig. 2) and S 892. x 2. 



9, 14, 16. — Posterior, dorsal, and ventral views of exterior. 



In fig. 14, a concentration of smaller costella; is 

 noticeable in the antero-median portion of the valve. 

 Such a concentration of ribs is a feature of these shells. 



1 0, 29. — Two views of a ventral interior, showing ponder- 



ous teeth and cavernous crural fossettes. 

 Silurian (Gotlandian), Klintehamn, Gotland, 

 Sweden. Cat. Nos. S 890 (figs. 9, 14, 16), and 

 S918. x2. 



Parmorthis crassicostata Schuchert and Cooper, n. sp. 



4, 5. — A rather heavily costellate species. 



Silurian (Niagaran), Martin's Mills, W. Tenn. 

 Holotype. Cat. No. S 91 3. x 2. 



Parmorthis waldronensis (Foerste) 

 7. — Ventral exterior of the common American shell that 

 has so long masqueraded under the name of Dal- 

 manella elegantula (Dalman). 

 Silurian (Niagaran), Jeflferson Co., Ind. Cat. No. 

 S 899. X 2. 



Isorthis perelegans (Hall) 



17, 18. — Dorsal and lateral views of exterior, the latter 



showing biconvexity of valves. 

 26. — Ventral interior, showing Schizofhoria-Y\ks muscle- 

 scars. 

 28. — Dorsal interior, showing cardinalia. 



Devonian (Helderberg), ClarksviUe, N. Y. Cat 

 No. S 1013. X 1. 



Figs. Isorthis rockhousensis (Dunbar) 



19. — Dorsal exterior. 



Low. Devonian (Rockhouse), Rockhouse, Tenn. 

 Cat. No. 9750 Y. P. M. x 2. 



Isorthis szajnochai Kozlowski 

 20. — Posterior portion of a dorsal valve, showing divergent 

 diductor tracks and median adductor fold. This is 

 very similar to the condition in Schizofhoria, see pi. 

 23, fig. 20. x2. 

 25, 30, 31, 33.— Ventral, dorsal, posterior, and lateral views 

 of a specimen representing the type species of the 

 genus. X 1.5. 

 27. — Dorsal interior, showing cardinalia and adductor field. 

 Fulcral plates visible just outside brachiophore plates. 

 Cf. corresponding view of Schizofhoria, pi. 23, fig. 

 21. X 1.5. 

 Up. Silurian (Borszczow), Wierzchniakowce, Po- 

 land. Cat. No. S2127. 



Isorthis arcuaria (Hall and Clarke) 



21. — Dorsal interior, showing elliptical adductor field 

 and prominent ridges of adventitious shell extended 

 around it. 

 23. — Ventral interior, showing prominent divergent diduc- 

 tor tracks and pallial sinuses extending from them. 

 Diductor tracks separated by a fold of shell which 

 bears adductor impressions. 

 Silurian (Niagaran), Decatur Co., W. Tenn. Cat. 

 No. S 1030. x2. 

 32. — Lateral view, showing strongly lenticular profile of 

 sheU. 



Silurian (Niagaran), Clifton, W. Tenn. Cat. No. 

 S1022. x2. 



Isorthis tetragonum (Roemer) 



22. — Dorsal internal mold, to show pallial markings. 



Devonian (Eifelian), Gerolstein, Germany. Cat. 

 No. S994. X 1.5. 



Isorthis canalicula (Schnur) 

 24. — Ventral interior, with musculature and pallial mark- 

 ings characteristic of the genus. 

 Devonian, Eifel, Germany. Mus. Comp. Zool., 

 Harvard Coll. x 2. 



Idiorthis matura McLearn 

 34. — Ventral internal mold, showing musculature. Cf. 



Wattsella, pi. 22. x 3. 

 3 5, 38. — Dorsal and ventral views of exterior, x 1.5. 

 36, 37. — Dorsal internal mold (37), x 1.5, and replica of it 

 showing strong median ridge and adductor field. 

 Cardinal process trilobed. 

 Silurian (McAdam), Arisaig, Nova Scotia. Cat. 

 Nos. 426 (figs. 34, 36, 37) and 426c Y. P. M. 



236 



