PLATE LXYI. 
1. Plagiostoma Rusticum. (Rude Plagiostoma .) Transversely oblong-ovate, oblique, smooth, convex; ears 
obscure, short ; surface, with about twenty-five, deep, strong, wavy, divergent, longitudinal sulci; beak some¬ 
what prominent. 
2. P. Concentricum. (Concentric Plagiostoma.) Obliquely elliptical, convex; hinge-line short, oblique* 
surface, with many, longitudinal, divergent striae, and a few, concentric lines of growth; beaks somewhat 
produced ; shell thick. 
3. P. Obscurum. (Obscure Plagiostoma.) Gibbose, sub-ovate; ears unequal, beaks produced; surface 
smooth, with many, fine, divergent ribs, and a few, concentric lines of growth. 
4. P. Elongatum. {Elongated Plagiostoma.) Transversely elongated ; acutely convex ; sides parallel; beaks 
sharp ; lower margin straight, short; surface, with a few, transverse sulci. 
5. P. Rigidum. {Rigid Plagiostoma.) Inflated, obliquely obovate; hinge-line long; ears narrow, equal; 
anterior side long, straight, concave ; posterior side rounded; beaks obtuse; surface, with many, sharp, thread¬ 
like, longitudinal ribs, with minute, intervening striae; beaks produced. 
(), P. Duplicatum. ( Double-plaited Plagiostoma.) Obliquely oboval, convex ; beaks produced ; surface, with 
many, sharp, divergent ribs, with a sharp, elevated line between each ; with about twenty-five ribs. 
7. P. Rigidulum. {Rigid Plagiostoma.) Elongated, sub-triangular; anterior side concave; posterior side 
rounded ; ears unequal; beak large, obtuse ; surface smooth, with many, oblique, rounded ribs. 
8. P. Ambiguum. {Ambiguous Plagiostoma.) Sub-triangular ; sides nearly equal; hinge-line short; beaks 
obtuse, produced ; surface, with about twenty-three, rounded ribs, crossed by a few lines of growth; margins 
scalloped. 
9. P. Pectinoides. {Pectinated Plagiostoma .) Oblong-ovate, compressed; beaks acute ; hinge-line long ; 
ears equal; surface, with about twenty, carinated ribs; intercostal sulci transversely striated ; margin scalloped • 
inside plain. 
10. P. Giganteum. {Gigantic Plagiostoma.) Obliquely-oblong, sub-compressed, deltoidal; umbones straight 
obtuse ; auricles small, anterior and longest one in large, broad, deep sulcus ; surface smooth, with obscure' 
longitudinal stria), crossed by a few, hollow lines of growth. 
11. P. Interstinctum. {Divided Plagiostoma.) Obliquely obovate; beaks produced; ears indistinct; surface 
with many, sharp, irregular ribs ; margin crenated. 
12. P. Hermani. {Herman's Plagiostoma.) Sub-rotund; anterior side straight; posterior side short ; ears 
unequal ; beaks slightly produced; surface smooth, with about fifteen, very flat, longitudinal ribs and a few 
lines of growth. 
13. P. Ovale. {Oval Plagiostoma.) Ventricose, elongated; sides equal, anterior one recurved ; ears large ; 
surface, with mai^, small, equidistant ribs ; intercostal sulci, with minute, transverse striae. 
14. P. Cardiforme. {Cardium-shaped Plagiostoma.) Nearly circular, inflated; ears equal; beaks prominent; 
surface smooth, with many, longitudinal sulci, crossed by transverse striae, scarcely visible except in the sulci 
where they look like minute punctures. 
15. P. Punctatum. {Punctated Plagiostoma.) See 19 below. 
16. P. Dubium. {Dubious Plagiostoma.) Obliquely ovate, anterior side nearly straight; posterior side 
rounded ; beaks obtuse; many flat divergent ribs, obsolete towards apex ; few remote lines of growth. 
17. P. Laeviusculum. {Smooth Plagiostoma.) Sub-triangular, oblique; hinge-line oblique, short, intercepted: 
ears small, unequal, longitudinally furrowed; surface, with many, longitudinal ribs, and many, concentric lines 
of growth ; margin scalloped. 
18. P. Hoperi. {Hoper's Plagiostoma.) Transversely and obliquely ovate; convex ; almost smooth ; surface, 
with slender, punctated striae; ears unequal, small, with longitudinal striae. 
19. P. Punctatum. {Punctated Plagiostoma.) Obliquely obovate, compressed; anterior side long; ears 
equal ; surface, with many, longitudinal, coarse striae, and very fine, transverse striae. 
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