PLATE LXI** 
1. Avicula Longiaxis. ( Lengthened Axis Avicula .) Valves equal; liiuge-line straight; anterior side acuminated; posterior side short ; 
surface, with fine, transverse strife ; shell thin. 
2. Pecten Gentilis. (Kindred Pecten.) Ovate, convex, smooth, with fifteen, slightly elevated, longitudinal, divergent ribs, alternately long 
and short; ears equal. 
3. P. Scalaris. ( Ladder Pecten.) 
4. P. Semicostatus. (Semi-ribbed Pecten.) Orbicular, convex; ears almost equal; posterior one square, anterior one more pointed ; surface, 
with many, rounded ribs. 
5. A. Antiqua. (Ancient Avicula.) Very oblique and arcuated ; auricles unequal; anterior one short, blunt; posterior one very long, acute 
at upper angle ; posterior side lengthened, with strongish, concentric ridges; anterior side short; beaks obtuse; surface, with imbricated stria). 
6. P. Ottonis. (Ottoman Pecten.) Sub-orbicular, convex, oblique ; surface, with many, longitudinal, radiating ribs, decussated by fine, con¬ 
centric strife ; ears unequal; margin crenated. 
7,8. P. Sexcostatus. (Six-ribbed Pecten.) Triangular, convex, with small, nearly equal ears; beak of larger valve incurved; both valves 
with six, longitudinal, divergent ribs ; margins crenated. 
9. A. Complicata. (Complicated Avicula.) Hinge-line oblique; right beak short, transversely ribbed, acute; left beak rounded; valves 
twisted, with longitudinal, nodulous ribs. 
10. P. Nexilis. (Wreathed Pecten.) Sub-orbicular, somewhat convex and inequilateral; ears large, radiated; surface, with fine, longitudinal 
divergent ribs. 
11. A. Papyracea. (Papyraceous Avicula.) Obliquely sub-ovate, compressed; valves flat; ears large, unequal, rectangular, with broad 
divergent stria), and close, longitudinal stria) on larger ear ; surface, with many, elevated stria), crossed by distant lines of growth. 
12. P. Valoniensis. (Valonian Pecten.) Sub-orbicular; ears nearly equal, blunt; larger one with radiating, small ribs, crossed by many, 
longitudinal strife; surface, with many, longitudinal, irrogular ribs. 
13. P. Transversus. (Transverse Pecten.) Transversely obovate; slightly convex; ears large, nearly equal; surface, with many, longitu¬ 
dinal ribs in threes, crossed by many, regular lines of growth. 
14. A. Longiarea. (Long-area'd Avicula.) Hinge-line lengthened, straight; anterior auricle long, broad, notched ; posterior auricle shorter 
narrow, acute ; valves inflated ; surface, with longitudinal, widish sulci; posterior side without sulci. 
15. Plagiostoma Brightonense. (Brighton Plagiostoma.) Obovate, compressed; posterior side with an ear; anterior side, concave 
lunulate, small; surface, with many, divergent, rounded ribs; margin crcnulated. 
10, 17. Pterinea Ventricosa. (Injlated Pterinea.) Obliquely elongated; ventricosc; hinge-lino straight, slightly oblique ; auricles unequal; 
beaks obtuse ; hinge, with oblique, narrow teeth, forming a triangular series, with a series of shorter teeth behind ; surface smooth. 
17*. P. Calvus. (Bald Pecten.) Orbicularly ovate, equilateral; convexo-plano, pellucid, shining; ears obtusely angular. 
18. P. Concentricus. (Concentric Pecten.) Elongated, smooth, with irregular, concentric ribs and depressed, divergent ribs; ears unequal • 
sides even. 
18*, 19. Monotis Decussata. ( Decussated Monotis.) Sub-orbicular; under valve convex, upper one flattish; hinge-line straight, horizontal • 
umbo rounded, blunt; surface, with many, divergent ribs, beset with imbricated spines; margins scalloped; upper valve, with large, posterior 
auricle ; anterior Bide, with small, acute ear; with many, undulating ribs from umbo. Length, J in.; breadth, £ in. 
Fig. 20 represents the hinge of Monotis SllbCOStata. 
21. Avicula Neptuni. (Neptune's Avicula.) Sub-orbicular, sub-compressed, hinge-line horizontal; anterior auricle short; posterior one 
large ; surface, with many, divergent, unequal ribs; Bides and base rounded. 
22. p. Radiata. (Bayed Pterinea.) Transversely oblong; surface, with a series of wide-set, longitudinal, divergent ribs, crossed by many 
waved strife ; margin scolloped. J ’ 
23. 24. A. Modiolaris. (Modtola-lihe Avicula.) Obliquely and transversely elongated; compressed; keeled towards beaks; auricles 
undefined; hinge-line straight. 
25. Posidonia Transversa. (Transverse Posidonia.) Transverse, oblique, compressed; beaks obtuse, rounded, placed near anterior margin; 
surface, with strong, transverse folds. 
2G. P. Truncata. (Truncated Posidonia.) Sub-quadrate; Bomewhat oblique; hinge-line straight, horizontal; anterior side short, narrow • 
posterior side, sloping obliquely from hinge-line; umbones obtuse; surface, with many, concentric ridges, with narrow sulci and minute, concentric 
strife. 
27, 28. A. Quadrata. (Squarish Avicula.) Sub-quadrangular, convex; hinge-line straight; anterior auricle small, rounded ; keel from beak 
to basal margin. 
29. A. Subradiata. (Sub-rayed Avicula.) Obliquely and transversely elongated; hinge-line horizontal; one valve flat, the other convex* 
posterior auricle, with a few, concentric lines; anterior side narrow, pointed ; posterior side smooth, concavish ; surface, with remote, radiating 
lines, crossed by concentric lines of growth. 
30. Gervillia Ineonspicua. (Inconspicuous Gervillia.) Transversely elongated; hinge-line whole length of valve; anterior side short 
rounded ; posterior side truncated; surface, with concentric wrinkles. * 
31. G. Minor. (Small Gervillia.) Very oblique; umbones flat; hinge-lino long, straight; surface, with many, concentric, strong wrinkles 
with smaller, intermediate ones. Length, g in.; breadth, } in. 
•^2. P. Minuta. (Small Posidonia.) Sub-triangular, inflated; umbo large, obtuse ; surface, with transverse wrinkles. 
33. P. Complanata. (Compressed Posidonia.) Inequivalve, transverse, oblong-ovate ; one valve a little convex, the other flat, and enveloped 
by the opposite one; hinge-line straight; beaks nearly obsolete; surface, with concentric lines of growth. Breadth, J in. 
34. P. Tuberculata. (Tuberculated Posidonia ) Sub-triangular, compressed, elongated; umbones central; surface, with concentric ridges 
crossed by three or four ribs, producing tuberculated appearance. 
35. P. Beeheri. (Becher's Posidonia.) Obliquely sub-triangular, compressed; with many, concentric ridges and fine strife ; umbones nearest 
anterior side. 
36. Pterinea Laevis, magnified. 
37. Ditto, natural size. 
38. P. Venusta. (Handsome Posidonia.) Transversely oval, concentrically wrinkled, striate ; ear large ; umbo short. 
41. Plagiostoma Mantelli. V. Plate LXI.*, Fig. 12. 
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