PLATE LXVI.* 
I, 2. Dianchora Striata. ( Striated Dianchora.) Triangularly ovate, oblique; beak prominent; free valve 
obscurely ribbed ; ears small, continuous along sides of valves. 
3. D. Lata. {Broad Dianchora .) Attached convexity moderate; semi-circular; beak of valve produced ; 
free valve plain ; surface, with obscure lines of growth, and a few, obsolete striae ; edge very sharp. 
4. Plagiostoma Spinosum. ( Spinous Plagiostoma.) Obovate; umbones central, blunt; one valve flat; 
surface, with many, flat ribs and deep sulci; margins crenulated ; convex valve, with curved, long spines ; 
whole surface, with fine, transverse striae. 
5. 6, 7. Avicula Inaequivalvis. ( Unequal-valved Avicula.) Deeper valve convex, enveloping smaller valve, 
with large, projecting auricle ; surface, with thirteen, narrow, rounded ribs, with striae between ; flatter valve a 
little convex, smooth, with divergent sulci; posterior auricles of both valves minute ; shell very thin. 
8, 9. A. Costata. ( Ribbed Avicula.) Deep valve transversely ovate ; auricles equal; surface, with eight, 
smooth, longitudinal, curved ribs, with internal sulci to correspond; shallow valve nearly flat, with many, 
external rays, posterior auricle of which separated from shell by deep, narrow sinus, and with a few, sharp teeth 
under auricle; opposite auricle large, elongated, acute. 
10. A. Lineata. (Lineated Avicula.) Obliquely ovate, compressed ; surface, with many, elevated lines ; 
anterior auricle minute ; posterior one triangular. Length, f in.; width, If in. 
II. A. Rectangularis. (Rectangular Avicula.) Smooth, sub-triangular, very convex; hinge-line long, 
straight; anterior side straight; posterior one produced in form of a lobe ; front rounded ; beaks acute. 
12. Anomia Radiata. {Rayed Anomia.) Flat; irregularly orbicular, with fine, radiating striae. 
13, 14. A. Laevigata. {Smooth Anomia.) Nearly circular, very thin, flat; surface smooth; umbo hardly 
elevated. 
15, 16. A. Convexa. {Convex Anomia.) Very convex ; beak large, prominent; surface smooth. 
17. A. Retroflexa. {Bent-back Avicula.) Short, broad, semi-circular, rather convex; transversely wrinkled; 
anterior ear minute. Length, £ in.; breadth, If in. 
18. A. Murchisoni. {Murchison's Avicula.) Obliquely elongated, inflated, smooth; with very obtuse, con¬ 
centric ridges ; beaks small, acute ; anterior auricle minute; posterior auricle not protruding beyond the margin. 
19. A. Modioliforme. {Modiola-formed Avicula.) Transversely elongated ; umbones on one side ; liinge-line 
undefined; with triangular ear on one side ; surface transversely wrinkled. 
20. A. Reticulata. {Reticulated Avicula.) Oblong-ovate, oblique; one valve convex; surface, with many, 
longitudinal ribs, decussated by strong lines of growth; auricles unequal. 
21. A. Orbicularis. {Orbicular Avicula.) Orbicular, convex, smooth; with a few, almost obsolete lines of 
growth ; beaks produced ; auricles small; hinge-line straight. 
22. A. Anomala. {Anomalous Avicula.) Obliquely elongated, disc flattened ; beaks acute ; hinge-line very 
oblique; surface, with many, longitudinal, narrow ridges, crossed by ill-defined lines of growth; valves very 
deep ; basal line sub-triangular. 
23. A. Obliqua. {Oblique Avicula.) Elongated, obliquely ovate, convex, smooth; hinge-line short; auricles 
undefined. Length, If in. 
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