316 



CRETACEOUS LAMELLIBRANCHIA. 



usually roimded, l)ut in one variety {WtJisteri, Mantell, Plate LTII, figs. 1, 2, 

 Text-figs. 71, 72) it possesses a sharp edge. When the folds are absent the 

 growth-rings become more regular. The curvature of the folds is usually small 

 in the more convex S})ecimens, but often greater in the less convex forms. The 

 curvature is often nearly symmetrical, but when the posterior ear is indistinctly 

 limited it tends to become unsymmetrical. 



The degree of development of the posterior ear varies considerably. In some 

 forms it is only indistinctly limited (Plate LIII, fig. 7, Text-fig. (38), and tlien the 



{' 



Fig. 74. — Inoceramus Lamareki var. Ciivieri, Sow. Upper Chalk (zone of Micraster cor-anguinum). Camp 

 Hill, near Salisbury. Dr. Blackuioro's Collection. Portion of a large left valve ; posterior and ventral 

 parts missing, x |. 



folds and growth-lines are continued with but little alteration in curvature on to 

 the ear. In other cases the ear is larger and more or less distinctly limited from 

 the rest of the valve (Plate LII, fig. 4, Text-figs. 03, (35, 66, 79, 81, 82) ; in such 

 cases the folds and growth-lines bend inwards at the limit, and the umbonal part 

 of the valve is often narrower and more acute. In a few large, flat forms, an 

 anterior ear is developed (Fig. 74). 



The anterior flattened area varies in size and in the distinctness of its 

 boundary. It may be nearly perpendicular to the plane between the valves 

 (Fig. 81), or may be slightly concave (Figs. G3, 66, 79), or slope outwards (Figs. 



