INOCERAMUS. 311 



Description. — Shell inequivalve, very inequilateral, of slight, moderate or 

 considerable convexity, sometimes inflated. Height greater than length. Hinge- 

 line of variable length in proportion to the height of the shell, forming more than 

 a right-angle with the anterior margin. Umbones terminal, curved inwards and 

 more or less forwards ; the left umbo more prominent than the right. Anterior 

 marginal part of valves flattened, more or less nearly perpendicular to the plane 

 of the valves, either limited by a sharp edge from the flanks or without a definite 

 boundary. Anterior ear developed in some flat varieties. 



The concentric folds may be absent, indistinct, or moderately or strongly 

 developed, with the dorsal and ventral slopes similar or with the ventral slope 

 steeper than the dorsal. Usually the folds are regular, but are not continued on 

 to the posterior ear. The curvature of the folds is often nearly symmetrical ; 

 its convexity on the convex and on some of the flat forms is small, but is greater 

 on the flat forms with a relatively short hinge. The growth-lines are distinct 

 and variable in number, and are sinuous where they pass on to the posterior ear. 



Remarks. — The forms included in this species show a great amount of 

 variation, and seem in that respect comparable with some species of Micraster and 

 Echinocori/s. Several of the varieties have been described as distinct species, but 

 the study of a large series of specimens has shown so many intermediate forms 

 that one can only regard the varieties as modifications of a very plastic species. 

 The features in which variation is most marked are the convexity of the valves, 

 the number, strength, and curvature of the concentric folds, the distance between 

 the growth-lines, the size and distinctness of the posterior ear, and the height of 

 the shell. 



Some forms of this species are only slightly convex (Plate LIII, fig. 7, Text- 

 figs. 73 — 83), so that in large specimens considerable portions of the shell 

 approach flatness. In other forms the valves are moderately or considerably 

 convex, and sometimes inflated (Plate LII, figs. 4, 5, Text-figs. 63—68). The 

 amount of convexity may remain nearly uniform throughout the growth of the 

 shell, or the earlier part may be only slightly convex and the later part very 

 convex — in such cases the early part resembles the adult shell of the large flat 

 varieties (Figs. 64, 65). The two valves may be of nearly equal convexity (Plate 

 LIT, fig. 5), or the left valve may be very convex whilst the right valve is only 

 slightly convex (Plate LII, fig. 6). 



The concentric folds vary in strength, number, form, curvature and regularity. 

 In the majority of cases the folds are prominent and form strong ridges (Figs. 68, 

 69, 78, 82, 84), but they may become only gentle undulations (Figs. 74, 77, 81), 

 and are sometimes indistinct or absent (Figs. 73, 75, 76, 79). The dorsal and 

 ventral surfaces of the folds may slope equally, or the ventral slope may be 

 steeper than the dorsal, giving a step-like appearance. The crest of the fold is 



