PLATE LI. 
1. Spirifer Ovalis* ( Oval Spirifer.) Elliptical; cardinal area triangular; umbones incurved; mesial fold large; 
with six or seven, broad ribs on eacli side. 
2, 16. S. Rhomboidea. ( Rhomboidal Spirifer.) Cardinal area wide; with smooth, longitudinal ribs. 
3. S. HumerOSUS. ( Hooded Spirifer.) Sub-cordiform; lower valve large; mesial fold produced; ridge of upper 
valve angular, elevated ; with many, longitudinal ribs. 
4, 5. S. Fusiformis. ( Spindle-shaped Spirifer.) Mesial fold ill-defined; cardinal area broad; with obtuse, longitu¬ 
dinal ribs and intervening sulci. 
6. S. Planata. {Plane Spirifer.) Sub-orbicular; cardinal area wide; umbones obtuse, remote; with many, plain 
ribs ; upper valve plain. 
7. S. Pyramidalis. {Pyramidal Spirifer.) Triangularly pyramidal; mesial fold narrow ; umbo of lower valve 
inflected ; base sharp ; with seventeen to nineteen longitudinal ribs. 
8. S. Integricosta. {Inter-ribbed Spirifei'.) Orbicular, inflated; umbones large, curved; mesial fold of medium 
size; with few, obtuse, longitudinal ribs. 
9. S. Trisulcosa. {Three-furrowed Spirifer.) Elongated, smooth; beaks'produced; cardinal area sub-triangular; 
upper valve with large, mesial fold and lateral plait on each side. 
10. S. Triradialis. {Three-rayed Spirifer.) Orbicular, smooth; beaks of lower valve large, incurved; upper valve 
depressed ; with broad, mesial fold and lateral fold on each side. 
11. 12. S. Gloveri. {Glover's Spirifer.) Circular, convex; both valves, with mesial sulcus; beaks produced; both 
valves with longitudinal striae, crossed by lines of growth; hinge-line short. 
13. S. DlipliciCOSta. {Double-ribbed Spirifer.) Elongated; umbones pointed; cardinal area wide; mesial fold 
angular; with many, longitudinal ribs; sides rounded. 
14. S. Linguilifera. {Tongue-shaped Spirifer.) Oblong, convex; mesial fold rounded; umbones produced; beaks 
obtuse ; lateral radiations and lines of growth obscure. 
15. S. Convoluta. {Polled Spirifer.) Elongated; cardinal area concave, with obsolete, remote stride, and obtuse, 
longitudinal ribs. 
17. S. Sexradialis. {Six-rayed Spirifer.) Oblong-ovate, smooth; cardinal area long; upper valve with broad, mesial 
ridge, and three, lateral ridges. 
18. S. Mesoloba. {Middle-folded Spirifer.) Sub-orbicular, smooth; umbo of larger valve produced; beak acute, 
inflected; cardinal area triangular; mesial fold broad. 
19. S. Tseniata. {Filleted Spirifer.) Orbicular, smooth; cardinal area narrow, transversely striated; mesial fold 
obsolete. 
20. S. OblatUS. {Raised Spirifer.) Gibbose, obovate, smooth; mesial rib depressed; beaks approximate; foramen 
of cardinal area triangular. 
21. S. Elliptica. {Elliptical Spirifer.) Transversely oval; beaks not much produced; mesial fold obtuse, broad; 
with wide rays and concentric striae; cardinal area flat, smooth. 
22. S. ObtUSa. {Obtuse Spirifer.) Gibbous; transversely ovate ; with longitudinal strias; beaks blunt, distant. 
23. S. Symmetrica. {Sijmmetrical Spirifer.) Sub-quadrate, gibbous, smooth; with wide, mesial fold; beaks 
incurved. 
24. S. Lata. {Broad Spirifer.) Gibbose, transversely elongated, smooth; mesial fold round, sub-depressed; with 
lateral radiation ; umbones blunt, short. 
25. S. Imbricata. {Imbricated Spirifer.) Short, transverse ellipsis; mesial fold obsolete; umbones approximate; 
with strong, radiating strife, crossed by concentric, imbricated laminfe. 
26. S. Decora. {Adorned Spirifer.) Orbicular, convex; umbones prominent; beaks acute; cardinal area triangular, 
wide; with obscure radiations. 
27. 28, 29. S. Glabra. {Smooth Spirifer.) Gibbous, smooth; umbones sub-acute; mesial fold rounded, depressed in 
middle ; margins sharp ; with indistinct lines of growth. 
30, 31. S. Filaria. {Threaded Spirifer.) Orbicular, flat; beaks small, contiguous; with fine, longitudinal strict, 
crossed by many lines of growth; with fine strife internally; hinge-line short. 
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