PLATE Lin. 
I, 2,3. Orthis Interlineata. ( Interlinear Orthis.) Transversely elliptical, compressed; beak prominent; smaller valve, with shallow 
depression ; base straight; with many, filiform, radiating stria) ; with sub-rostral plaits arcuated to include cordiform area. 
4, 5. 0. PennatUS. ( Unequal-winged Orthis.) Semi-ovate, oblique, inflated; hinge-line parallel; with many acute, radiating ribs, with two 
or three remote lines of growth ; margins crenated. Length and breadth, J in. 
6,7, 8. 0. Orbiculata. ( Orbicular Orthis.) Orbicular; with fine, radiating stria); larger valve convex; beak curved, with straight, longi¬ 
tudinal ridge internally and curved one on either side ; smaller valve convex, with central depression ; hinge-area minute. Length and breadth, $ in. 
9,16. 0. Parallela. (Parallel Orthis.) Oblong-oval, compressed; hinge-lino narrow; base wide; deeper valve, with tumid, mesial line; 
flatter valve, with hollowed, mesial depression ; with many radiating stria?, crossed by lines of growth ; sub-rostral ridges of deeper valve parallel. 
10. 0. Radians. (Radiated Orthis.) Semi-cylindrical, compressed ; base concave ; with about fifteen, sharp plaits, beaks produced. Length 
$ in.; breadth, } in. 
II, 12. 0. Tenuistriata. (Thin-Striated Orthis.) Semi-circular; base straight; with many, fine stria?; one valve convex, the other flatter; 
hinge-area narrow, parallel. 
13,14. 0. Alata. (Winged Orthis.) Semi-circular; hinge-line parallel, with cuspidate, auriform processes; with about eighteen radiating 
plaits ; sides crenated. Length, \ in.; breadth, } in. 
15. 0. Granulosa. (Granular Orthis.) Orbicular, compressed, lenticular ; with many granulated stria). 
17. 0. Rustica. (Rude Orthis.) Transversely Bub-quadrate, depressed; hinge-area large, triangular; with about forty to fifty ribs ; base 
straight. Length, 1 in. ; breadth, 1J in. 
18, 19. 0. Alternata. (Alternately-ribbed Orthis.) Transversely, obovately sub-quadrangular; with numerous, raised stria). Length, 2 in • 
breadth, 1^ in. ’ 
20. 0. Compressa. (Compressed Orthis.) Semi-ovate, lenticular, compressed; hinge-line parallel; with many, punctuated stri®. Length 
1 in.; breadth, 1J in. ’ 
21. 0. Protensa. (Stretched-out Orthis.) Semi-ovato, depressed; unequally thick; with sharp, divergent stria), crossed by a few remote 
lines of growth. Length, § in.; breadth, £ in. 
22 . 0. Callactis. Circular; compressed ; with about twenty radiating ribs. Length, £ in.; breadth, j in. 
23. 0. Arcuata. (Arcuated Orthis.) Transversely oblong-ovate, compressed; one valve convex, the other with mesial sulcus; with fine 
divergent stri®. * 
24. 25. 0. Antiquata. (Antiquated Orthis.) Semi-circular, compressed ; hinge-area narrow; smaller valve flat; with many radiating stri® 
crossed by remote lines of growth. Length, J in.; breadth, | in. 
26. 0. Expansa. (Expanded Orthis.) Semi-circular, compressed; hinge-line parallel, internally plaited; base rounded. Length, 11 in • 
breadth, 2 in. * 
27. 0. Subarachnoidea. (Spider Web-like Orthis.) Elongated ; hinge-line parallel; lesser valve convex; with many subdivided stri®. 
28. 0. Anomala. (Anomalous Orthis.) Longitudinally oblong; convex; umbones obtuse; sides straight; with many longitudinal stri®- 
base rounded. Length and breadth, 1J in. * 
29. 0. Lata. (Wide Orthis.) Transversely semi-cylindrical, depressed ; base straight; sides rounded ; with linear, divergent stri®. Lencth 
4 xn. ; breadth, $ in. ® * 
Semi-circular, convex; base rounded; with about thirty radiating stri®; umbo 
0. Semicircularis. (Semi-circular Orthis.) 
Length, £ in.; breadth, £ in. 
0. Umbraculum. (Shaded Orthis.) Semi-circular; hinge-line parallel; lower valve convex towards beak ; cardinal area low, strongly 
many, 
(Inter-ribbed Orthis.) Orbicular ; hinge area triangular ; with filiform, divergent ribs. 
(Lens-shaped Orthis.) Sub-orbicular, compressed ; valves convex ; lower valve, with mesial sulcus near umbo; with about 
30, 31. 
protruding. 
32, 33. 
sulcated ; upper valve flat, with low, triangular, cardinal area; with twelve or thirteen, filiform cost® from umbones, with many stri®. 
34, 35, 36. 0. Canalis. (Canalled Orthis.) Longitudinally semi-ovate ; one valve convex, beak produced, incurved ; other valve flattisli with 
central sulcus; base pointed, depressed ; with fine, radiating stri®. Length and breadth, J in. 
,. 37 * P* Longisulcata. (Long-furrowed Orthis.) Transversely elliptical, compressed; sides rounded; hinge-lino arcuated; with 
divaricating stri®, crossed by concentric lines of growth ; internal plates divergent. 
38. 0. Actoniae. (Acton's Orthis.) Transversely obovate; one valve convex, the other flat; with about fourteen, radiating ribs, quadrifid 
or tnfid at outer extremities. Length, $ in.; breadth, 1 in. 
39,40. 0. Bilobata. ( Two-lobed Orthis.) Transversely sub-quadrate; hinge-line parallel; upper valve concave, with broad, longitudinal 
elevation ; lower valve convex, with deep, broad, longitudinal channel; with many, radiating ridges. Length, ljl in.; breadth, 1^ in. 
41. 0. ConcentPica. (Concentric Orthis.) Semi-circular, compressed ; hinge-line parallel; with internal plaits; with divergent, filiform stri®. 
42. 0. Filosa. (Threaded Orthis.) Semi-oval, thin, flattened; beaks hardly elevated; sides straight; with many, filiform, radiating stri® 
Length and breadth, J in. 
43. o. Intercostata. 
44. 45. 0. Lens, 
twenty undulated stri®. 
i 4C>. 0. Cancellata. (Cancellated Orthis.) Suborbicular, compressed; umbones produced; with many, radiating ribs; with four or five 
longitudinal stri® ; the whole crossed by many, concentric stri®. 
breadth ^l'j ^ u ^^ urca ^ us ' (Many-forked Orthis.) Fan-shaped, convex; base rounded; with many radiating forked ridges. Length, 1 in. ; 
48. 0. Interstrialis. (Interstriatcd Orthis.) Semi-circular, hinge-line parallel, with sub-auriform processes; one valve convex, the other 
depressed; with many, radiating stri®. ° r * uer 
49. 0. Parallela, (Parallel Orthis.) V. Figs. 9, 16. 
oO. 0. Costata. (Ribbed Orthis.) Semi-circular; one valve conical; hinge-line arcuated; umbones produced, with triangular area beneath ; 
sides^angular ; with about twenty, radiating ribs. Length, \ in.; breadth, \ in. 
51, 52. 0. Semilunata. (Semilunar Orthis.) Transversely sub-quadrangular ; with deep, longitudinal stri® ; beaks not prominent; hinge- 
area small ; lesser valve, with longitudinal, central depression ; internally with two, semi-circular ridges. Length and breadth, \ in. 
53, o4. Leptsena Reticulata. (Reticulated Lcptana.) Sub-ovate, sub-compressed ; hinge-line parallel; sides straight; base rounded ; with 
strong stri®, crossed by concentric stri®. 
f ? P m (Spur Orthis.) Scmi-circular; with about twelve, granulated, divergent ribs, forming fringe all round. 
«j 6 , o7, 58. 0. Lunata. (Moon-shaped Orthis.) Semi-oval, convex ; with many, fine stri®, crossed by equidistant lines of growth ; one valve 
flat. Length, 1 $ in.; breadth, 2 in. 
breadth Hybrid* 1 * (Hybrid Orthis.) Orbicular; lenticular; valves equal; base straightish; with many, radiating stri®. Length and 
62,63. L. Prcelonga, (Lengthened Leptccna.) Transversely obovate, convex, with central sulcus; beak of larger valve ventricose ; smaller 
valve concave; margin deflected ; with coarse, transverse stri®. 
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