PLATE 28 



Figs. Conchidium biloculare Linnaeus 



1,5, 6. — Dorsal, ventral, and lateral exteriors of a small 

 individual. 

 3. — Posterior view. 

 8, 11, 12, 18. — Dorsal, ventral, lateral, and posterior views 

 of a large specimen. Posterior view shows a remnant 

 of the deltidium in place. Ventral view shows a 

 long crack, marking position of median septum. 

 9. — Dorsal tiew, showing nearly complete deltidium in 

 place. 

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



51859. X 1. 



2. — Internal mold seen from dorsal side. 

 23. — Specimen broken in plane of median septum. Long 

 brachial process visible on the right. 

 Silurian (Gotlandian), Hejde, Gotland, Sweden. 

 Cat. No. S 1867. x 1. 

 4. — Small individual in dorsal view, to show uncovered 

 delthyrium. 

 26. — ^Apical portion of dorsal valve with internal plates. 



Silurian, Klintehamn, Gotland, Sweden. Cat. No. 

 S1872. xl. 

 1 0. — Interior of apical portion of dorsal valve. 

 20. — Spondylium of ventral valve. 



Silurian, Gotland, Sweden. Cat. No. 7839 Y. P. M. 

 xl.5. 



Conchidium tenuistriatum Walmstedt 



17. — Exterior of large specimen, in dorsal view. 



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



51860. xO.S. 



Lissocoelina pergibbosa (Hall and Whitfield) 



7, 14. — Dorsal and lateral views of an internal mold, the 

 former showing dorsal plates and the latter the unusual 

 gibbosity. 

 Silurian (Niagaran), Cedarville, Ohio. Cat. No. 

 S1834. X 1. 



Lissocoelina ? maquoketa (Hall and Clarke) 

 13, IS, 16. — Ventral, dorsal, and posterior views of this 

 gibbous pentamerid, showing internal septa. This 

 form must be excluded from Harfidium because of 

 the short median septum. 



Silurian (Niagaran), Maquoketa, Iowa. Cat. No. 



S1840. 



Figs. Lissocoelina ? maquoketa (Hall and Clarke) — Cont. 



2 1 . — Lateral view of an unusually gibbous form. 



Silurian (Niagaran), Maquoketa, Iowa. Cat. No. 

 S 1836. 



Pentameroides subrectus (Hall and Clarke) 



19. — Posterior view of an internal mold, showing septa. 

 Note single septum in dorsal valve. 

 Silurian (Clinton), Stony Creek, Ontario. Cat. 

 No. S 1842. X 1. 



Pentameroides, n. sp. 

 22. — View of dorsal exterior, showing single median 

 septum. 

 Silurian ("Stricklandia-Pentamerus" zone of the 

 Clemville), Black Point, New Brunswick. Cat. No. 

 S1841. X 1. 



Brooksina alaskensis Kirk 



24. — Internal septa of a specimen broken through the 

 middle. 

 Silurian, S. E. shore Kosciusko Island, Davidson 

 Inlet, S. E. Alaska. Cat. No. S 2223. x 1. 



Stricklandia gaspeensis Billings 



25. — Posterior of dorsal valve, showing brachial processes. 

 This structure is more nearly rhynchonelloid than 

 pentameroid. Davidson figured the brachial process 

 affixed to the extremities of the lateral plates bound- 

 ing the notothyrium. 



Silurian (La Vieille), La Vieille Cove, E. of Gascons, 



Quebec. Cat. No. S 1899. x 2. 



27. — Posterior of ventral valve, showing cardinal area and 

 fractured spondylium. 

 Silurian (Clinton), S. W. corner of Barachois at 

 Port Daniel, Quebec. Cat. No. S 1887. x 1. 



Stricklandia lirata (Sowerby) 



28. — Dorsal exterior. 



Silurian, Gotland, Sweden. Cat. No. S 1883. x 1. 



250 



